There haven't been many constants for the Twins over the past six years. A look back at the 2004 roster seems like a study in ancient history. Doug Mientkiewicz, Luis Rivas, Cristian Guzman and Corey Koskie manned the infield. Michael Cuddyer had not yet established himself as a regular. Joe Mauer entered the season without a major-league at-bat to his credit. Johan Santana was just finally becoming a full-time member of the rotation.

Yes, it was a different era in Twins baseball. But it was also the year Joe Nathan, who'd been acquired from the Giants during the previous offseason, took over as the team's closer. And in the six years since, he has been one of the most consistent forces in baseball, annually posting elite numbers from the closer position while remaining remarkably healthy and durable.

Between 2004 and 2009, Nathan never appeared in fewer than 64 games and never pitched fewer than 67 2/3 innings. He never posted an ERA higher than 2.70, and never posted a WHIP higher than 1.01. He's had his blips from time to time, but for the most part Nathan has been a model of consistency.

In those six years, he racked up a whopping 246 saves, the highest total of any closer in baseball during that span. That figure places him just eight short of the Twins' all-time saves leader, Rick Aguilera, so Nathan seemed certain to claim his place in the franchise history this year.

Now, that won't happen this season. And it might not happen at all.

After experiencing pain in his elbow during his first spring training appearance last week, Nathan flew back to Minnesota, where imaging scans revealed a "significant tear" in his ulnar collateral ligament. While he and the team have suggested that the closer will wait a couple weeks for the swelling to go down before making a decision on his future, there is almost no chance that Nathan will avoid Tommy John surgery. At the age of 35, bouncing back from this serious procedure will be difficult for Nathan. Not only is his 2010 season likely done, his future as a major-leaguer could be in grave doubt. That's pretty heartbreaking, not only because the Twins owe him $24.5 million in guaranteed money on his current contact, but also because he is truly one of the good guys in baseball and one of the last remaining links to that previous era in Twins history.

For now, though, the Twins can't afford to look at the big picture and sulk. They'll need to determine who will pick up the slack this year in the absence of Nathan, who has converted 91 percent of his save opportunities since taking over closing duties in '04. While a closer-by-committee approach based on match-ups and who's available may be the most desirable solution, La Velle suggests that the Twins are unlikely to follow this path and my guess is that he's right. Ron Gardenhire will likely try to settle on an individual replacement for Nathan who can regularly be called upon in most instances to close out the ninth inning when the Twins lead by three or fewer runs.

Nathan has been one of baseball's elite closers over the past several years and no one the Twins can call upon internally will come close to filling the void he leaves in the bullpen. Yet, in a sense, the timing of this injury is as good as it possibly could be. The Twins enter this season with more relief depth than they've had in past years, and the injury comes early enough in spring training that they'll have plenty of time to weigh their options and formulate a plan before the season starts. There are a number of guys who could be called upon to overtake the closer reigns for the 2010 season, so I'll break down the candidates in order of likelihood of winning the job:

1. Jon Rauch
Rauch possesses many of the traits Gardenhire is likely to look for in a replacement closer. For one thing, he's a veteran, with 356 major-league appearances spanning seven seasons. He also has closing experience, having filled that role with the Nationals for a period of time during the 2008 campaign. After joining the Twins last year, he posted closer-type numbers, posting a 1.72 ERA while striking out 14 batters over 15 innings. Yet, that small sample hides the fact that Rauch's strikeout rate dropped overall last year (he fanned just 35 batters in 54 1/3 innings in Arizona prior to being traded) and that he has a tendency to get hit hard, having allowed a .425 slugging percentage while coughing up 17 homers over the past two years. Tall, tattooed and mean-looking, Rauch has the imposing look of a closer and seems the favorite to get save opportunities early in the year. There's no guarantee he'll stick in that role, though.

2. Jesse Crain
Those who recall watching Crain pitch over the first half of last season -- when his performance got so bad that he was demoted to the minors -- might be a bit baffled to see him rank second on this list. But after being recalled in July, Crain posted a 2.91 ERA while holding opposing hitters to a measly .217 batting average and .292 slugging percentage. While he's not a strikeout artist, he has proven capable of missing bats (7.3 K/9 over the past two seasons) and he had plenty of closing experience in the minors, where he was groomed to become a late-inning dominator. If Crain can pick up where he left off last season, he'll be the best candidate to fill in for Nathan. Of course, given his inconsistency since returning from shoulder surgery, that's no given.

3. Pat Neshek
When he first broke into the league, Neshek was about as dominant as Nathan. In 2006, the quirky right-hander posted a 2.19 ERA and 53-to-6 strikeout-to-walk ratio over 37 innings. He carried that success forward into his first full season with the Twins, dominating as eighth-inning setup man during the first half of the '07 campaign, but he wore down late in the year and the next spring he felt his UCL go pop, requiring Tommy John surgery (sound familiar?). Neshek hasn't pitched in a big-league game since early 2008 and we all know how difficult the initial return from TJ surgery can be (paging Francisco Liriano), so he seems like a longshot to even make the Opening Day roster this spring. Still, he's looked sharp in exhibition play and it would be no surprise to see him get a shot at closing out games somewhere along the line.

4. Matt Guerrier
Guerrier was Mr. Eighth Inning last year and he handled the job well, posting a 2.36 ERA and 0.97 WHIP while effectively bridging the gap to Nathan on a regular basis. This would suggest that he ought to be first in line to step in for the fallen closer. Yet, Guerrier seems perfectly suited for the role he has been in for the past couple years. His durable arm has racked up more appearances over the past three years than any other big-league pitcher, and as a guy who used to start and work in long relief he's capable of tossing multiple innings. Certainly, Guerrier has the resume to suggest he could be effective in a ninth-inning role, but I think the Twins would be doing themselves a disservice by removing him from his current spot. I suspect Gardenhire would feel the same way.

5. Francisco Liriano
Given that he has some of the filthiest stuff of anyone on the staff, Liriano has frequently been brought up as a candidate to move to the bullpen, where he ostensibly could unleash the full power of his arsenal in shorter stints. Indeed, Liriano's mid-90s fastball and dirty slider could make him a natural fit in the closer spot, but if he has truly regained his confidence and command he's too much of an asset in the rotation to be removed. If Liriano experiences problems similar to the ones that plagued him last year and is wearing down by the fifth inning in his starts, we could see the Twins give him a shot to close out games. But I'm hoping that won't happen.

6. Jose Mijares
In 72 big-league innings, Mijares has registered a 2.12 ERA while holding opposing hitters to a .206 batting average -- very nice numbers. However, Gardenhire requires a situational left-handed specialist and Mijares fits that role perfectly with his tremendous numbers against lefties (not to mention his mediocre numbers against righties). He could get some chances to pitch the ninth when opposing lineups are sending a couple left-handed hitters to the plate, but outside of that I'd be extremely surprised to see him notch many saves this season.

After Nathan's struggles in the playoffs last year, I was frustrated by fans who focused on his late-season issues while overlooking how amazingly well he'd pitched over the first five months of the season, not to mention his previous five years in a Twins uniform. Now, with Nathan gone, those same fans will come to realize what they'd been taking for granted in their rants. For the vast majority of his career in Minnesota, Nathan has been close to automatic in the ninth inning. And while I'm confident the pitchers above can do an adequate job of replacing his contributions, none of them will be able to fully fill his shoes. Panicky fans who are suggesting that Nathan's loss suddenly makes the Twins a far worse team are grossly exaggerating the effect of a dominant closer on the win/loss column, but there Nathan's absence will be noticeable.

Sometimes you don't miss a good thing until it's gone.