Vikings head coach Mike Zimmer loves challenges. None will be bigger defensively than figuring out what to do at linebacker.

It's a position that will see two different starters paired up with Chad Greenway. On paper, it's the biggest "unknown" position group and probably the weakest at this point. There are a number of unproven linebackers in position to start with Greenway. Outside of the battle at quarterback, the situation at linebacker is the second biggest headline heading into training camp.

On the eve of training camp, we break down the current situation at linebacker (Here's our defensive line preview from Tuesday, in case you missed it.)

WHERE THINGS STAND: The Vikings released linebacker Erin Henderson and opted not to bring back Marvin Mitchell. Both linebackers started in Week 1 last season alongside Greenway. In a weak crop of linebackers in both free agency and the draft, the Vikings didn't make a big splash acquisition that could immediately fill a need.

They brought back Jasper Brinkley to one-year deal and also signed Dom DeCicco and Mike Zimmer. In the draft, the Vikings selected Anthony Barr with the ninth overall pick and took Brandon Watts in the seventh round.

The new faces blend with some unproven linebackers already on the roster, specifically Audie Cole, Gerald Hodges and Michael Mauti. Zimmer and linebackers coach Adam Zimmer will attempt to make the most of pieces they have available, which will be a difficult task to sort out over the next few weeks.

CAMP BATTLE TO WATCH: Every job is open. Again, every job is open.

It's difficult to even claim whether any linebacker has separated himself up to this point because they've been rotating constantly so the coaching staff can get a better feel for every player before training camp. We've seen even Greenway get some reps at middle linebacker and there's a possibly he could be used at the position during the regular season.

It's tough to see Barr start on Week 1 unless he makes a tremendous amount of progress over the next month that the Vikings would feel comfortable using him in any situation. At this point, Barr is a naturally gifted pass rusher with his size, speed and strength that could use more time understanding the position. Cole flashed his potential filling for Henderson at middle linebacker late last season. Brinkley has the tools that fit this defense as a run stopping linebacker, while Mauti and Hodges still remain as unknowns with potential.

THE BURNING QUESTION: Are Greenway's best days behind him? He was ranked as the second to worst 4-3 outside linebacker out of 35 players last season by Pro Football Focus. Though, it didn't take a complicated formula to see that Greenway regressed last season. He played through a fractured wrist for the final portion of the season but looked a step slower. Greenway will turn 32 in January heading into the final year of his deal, with an unguaranteed base salary of $7 million. This year is a big opportunity to prove his worth either for the Vikings, or another team.