McLeod Bethel-Thompson is usually the last Vikings player to leave the field after practice. As players and coaches stream into the locker room, Bethel-Thompson stays behind to throw passes to anyone willing to catch them. He’s also one of the first players to arrive at Winter Park every morning and among the last to leave.

“He’s one of those guys you would refer to as a gym rat,” offensive coordinator Bill Musgrave said.

Bethel-Thompson, the No. 3 quarterback, has a simple explanation: “I’ve got to get better.”

Bethel-Thompson should get ample work in the Vikings’ final preseason game Thursday night against Tennessee. Christian Ponder won’t play and Matt Cassel might get limited snaps, giving coaches an opportunity to evaluate Bethel-Thompson extensively.

The fourth preseason game offers bubble players one last chance to impress before final cuts, which come Friday and Saturday. Bethel-Thompson wants to prove he deserves a spot on the 53-man roster.

As Vikings fans fret over Ponder’s long-term viability and debate whether Cassel is an upgrade, Bethel-Thompson toils as an intriguing project with a cannon arm whose future in the NFL remains an open-ended question.

Inactive for all 16 regular-season games in 2012, Bethel-Thompson served as Joe Webb’s backup in the playoff game against Green Bay with Ponder sidelined because of an arm injury. There’s speculation that Webb’s bid to make the team as a wide receiver this season could impact Bethel-Thompson’s fate. In theory, the Vikings don’t need a third quarterback if they keep Webb, because he could play quarterback in a pinch.

Vikings coaches, however, seem encouraged by Bethel-Thompson’s progress. The young quarterback said he feels “light years ahead” of his rookie season, but … “I don’t think I’m even close to what I can be,” he said.

Bethel-Thompson has the strongest arm of the Vikings quarterbacks by far, but he’s still learning the finer points of the position, including how to throw passes with touch instead of constant fastballs.

“We all like to watch a guy who can throw the ball deep,” quarterbacks coach Craig Johnson said. “But there’s more to the game. He has a naturally gifted arm, hard worker, the whole nine yards. But he just has to learn how to slow down a little bit because he gets a little excitable.”

That’s understandable considering his unconventional path. Injuries limited Bethel-Thompson to only 24 games in college — not all of them as a starter — split between UCLA and Sacramento State. He has logged time in Arena Football, United Football League and the Miami Dolphins practice squad.

“I’m so happy and so lucky and so blessed to be able to play this game that I would still be playing if I was in the Arena League making $200 a week,” he said. “If Coach told me I had to run a mountain to be the starter, I’d go run a mountain.”

No one questions his dedication, but the direction his career takes will hinge on his ability to refine his game to complement his strong arm. He spent hours this offseason throwing footballs into buckets, a drill designed to improve his accuracy and touch.

Johnson noted that a strong arm can be a “curse” sometimes because young quarterbacks have a tendency to become overly confident in their ability to squeeze passes into tight windows.

“I’ve never met a big-armed guy who has not had that mentality,” Johnson said. “What you have to guard against is, while I do have a big arm, I just can’t rely on my arm strength.”

Bethel-Thompson is striving to find that balance while also understanding that reps in practice are minimal for the No. 3 quarterback. That’s the biggest challenge, he said, which is why he stays late after practice.

Johnson said coaches want to see how Bethel-Thompson handles himself Thursday night. They’re evaluating whether he can get the offense into the right plays and protections and show command of the system.

“We’ve seen the flash,” Johnson said. “Now we’re just trying to work on the consistency.”