Sean Harlin has lived a nice, serene existence for nearly a decade now, a comfy chair in a dark room with a ballgame on the TV in front of him. His job has been sort of quasi-librarian, cataloguing every pitch, every swing, every inning of every Twins game since 2006, so ballplayers can diagnose their own deficiencies or scout the other team’s tendencies. Watch the game, log it all, move on to the next one, all in relative anonymity.

All that changes for Harlin on Monday. Big-league pressure arrives on Opening Day.

“I already told Sean, he’s going to be the first person I bury if something goes wrong,” Twins manager Ron Gardenhire said, presumably in jest. Presumably. “I told him [he’s going] right under the bus.”

More like, right under the TV truck. After decades of defending umpires and their inevitable mistakes as part of the game’s “human element,” and 28 years after the NFL introduced video replay in 1986, America’s oldest professional sport finally has embraced 21st century technology and spent millions installing it in all 30 ballparks. Beginning this season, umpires’ calls will be subject to review by their peers, using high-definition digital replays to uphold or overturn controversial decisions.

And while the new replay-challenge system might put the focus on managers and umpires, one group appealing the other’s mistakes to a higher court in New York, the real pressure will be on guys like Sean Harlin.

“Get it right, get it fast, get it to the manager,” said Harlin, the team’s director of major league video. “They’ve put a lot of trust in me, so I’m excited about the challenge.”

Baseball tested the new rules during spring training games, albeit with inferior equipment and no central command center to review video replays — nothing like the systems MLB installed in each ballpark, at a reported $10 million price tag. The practice helped players, managers and fans become familiar with the basic procedures:

• Each manager may challenge one call per game, and if a call is overruled, he may challenge a second. But that’s the limit. From the seventh inning on, umpires may initiate a review of their own, if a team has used its allotment of challenges.

• Decisions will be handed down by an umpire monitoring the game from MLB’s new video command center in New York, a dark, windowless room full of high-tech monitors (MLB won’t reveal its cost) that looks like a NORAD bunker for directing U.S. air defense. On-field umpires will rotate through the season to replay judges, with every camera angle available to him for multiple slow-motion viewing, if necessary. The umpire will communicate his decision — video confirmed the call was correct, proved conclusively that it was wrong, or was inconclusive, so the call stands — to the crew chief via a headset near home plate, a process that baseball hopes takes only 60 to 90 seconds.

• Managers must let the umpire know in a timely manner that he is challenging a call, as quickly as 30 seconds after the play if it’s inning-ending.

And that last bit is where Harlin comes in. Managers might feel that an umpire’s call is wrong, but “we’re a long way from the play, and it’s not always a great view,” Gardenhire said. To help managers decide whether to challenge a call, each team is allowed to have one person watch the game on video and communicate with the dugout. Coaches and players are eligible, but the Twins, like most teams, chose the person most experienced at examining video.

The clock is ticking

That experience is going to be critical, because Harlin will have even less time to determine whether there is evidence of a missed call than the umpires making the ultimate decision. The replays might include super-slo-mo, but his decision will have to be 100-gigabyte fast.

“I’m sure the technology is cutting-edge, but it still comes down to my judgment, and seeing what you have to in a hurry,” Harlin said. “It’s one thing to look at it over and over and say, ‘OK, I’m sure he was out.’ But if it takes you two or three minutes, that’s not helpful.”

To buy time, Gardenhire expects he will emerge from the dugout and walk over to the umpire — “I’m used to running out there with a red face, so it’s going to be different,” he joked — and essentially stall, maybe ask the umpire what he saw, until Harlin relays word to coach Paul Molitor on the bench whether there is sufficient evidence to overturn the call, which Molitor will signal to the manager.

(Harlin might not be the only one with the ability to decide whether a challenge is warranted. In another policy change that’s certain to be popular, MLB has lifted its ban on showing replays of close or controversial plays on the stadium scoreboard.)

While Gardenhire chats with the umpire, Harlin will operate the controls of the Hawkeye system, a derivative of the super-accurate cameras that professional tennis uses to rule on close calls, which MLB is installing in every home and road clubhouse in the league. He will have access to every camera angle from both teams’ feeds, and can replay each one, whether or not the broadcast uses it.

Trouble is, baseball demonstrated the Hawkeye console, controlled via keyboard and mouse, for video coordinators during spring training, but Harlin won’t operate it himself until Monday.

“I know how it works, but I’ve got to get up to speed in a hurry,” he said. “It’s going to take some getting used to — seeing all those screens, knowing which ones to look at again, making a decision in a few seconds.”

Harlin certainly is used to watching baseball on a video monitor. Excluding the birth of his son and his father’s funeral, the Eagan native, a Twins employee for 23 years including more than a decade as media-relations manager in the Metrodome days, has watched every pitch of every Twins game since becoming video coordinator in 2006 — yet he never has seen one of them live.

“I hear Target Field is pretty nice,” he joked from his office deep inside it, just off the home clubhouse. “I’d like to see a game there someday.”

Practice time is over

That’s off in the future, though, as he helps Minnesota adapt to baseball’s new era. He has worked with Gardenhire, Molitor and the coaching staff to prepare for the changes, and he is not the only one.

Former major league manager Joe Torre, now MLB’s executive director of baseball operations, is in charge of implementing the system, and he sent another Hall of Fame manager, Tony La Russa, to each spring camp to explain the system to managers and answer their questions.

“They’ve made it clear that this is the starting point, that they don’t expect it to be perfect right from Day 1,” Gardenhire said after his meeting with La Russa. “They’re going to make changes to the system once we see how well it works.”

The replay rules were in effect in a handful of spring games to get teams and umpires used to it, but Harlin was limited to watching the broadcast of the game, usually with only two or three cameras — and not always high-definition — with no ability to cue up replays on his own.

“We practiced the procedures, which was good. But it wasn’t what it’s going to be like during a real game,” he said. “I’m planning to check every play for a while, just to become fluent in the process.”

It became apparent during spring training that umpires will be generous about consulting replays from the seventh inning on, just to prevent being embarrassed by a missed call that isn’t corrected. Having an umpire put on a headset might soon be as ubiquitous as having basketball referees going to video monitors to review out-of-bounds and timing issues during NBA and NCAA games.

“I think they want to get it right, so they’ll be under the gun a little bit. They’ll say, I don’t want to be on ESPN tonight, so let’s check it,” Gardenhire said. “But it’s going to go fast. They’ll have a decision as soon as they put on the headset, right away.”

That’s the speed Harlin is hoping for, too. He knows his job has changed, from monitoring a game to perhaps helping to decide its outcome.

“Somebody tweeted that video coordinators may now be the difference between winning or losing. I’m actually excited about that, excited that I can contribute, even if all I’m doing is providing information so Gardy can make a decision,” Harlin said. “For [General Manager] Terry Ryan and Gardy to have that faith in me, it’s really an honor.”