The Vikings enter the 2018 offseason with plenty of uncertainty at quarterback. Minnesota coach Mike Zimmer knows it.

Neither 2015 starter Teddy Bridgewater, 2016 starter Sam Bradford nor 2017 starter Case Keenum is under contract for 2018.

"It's really the $64,000 question for all three of those guys. Can Sam stay healthy, can Teddy get back to where he was, can Case do what he did again," Zimmer said Thursday at the NFL Combine in Indianapolis. "We have a lot of question marks."

Bridgewater was a first-round pick of the Vikings in 2014 and was named the NFL Rookie of the Year. Following a Pro Bowl season in 2015, he entered 2016 with a lot of optimism, but a career-threatening knee injury cost him the 2016 season and most of 2017.

"I love Teddy. I've said that before. He is a competitor; he's got the heart of a Lion," Zimmer said Thursday. "I'm hopeful for Teddy. I hope he has an outstanding career and I hope that it's with me."

Following Bridgewater's knee injury, the Vikings traded for Bradford, who went on to lead the NFL with a 71.6 percent completion rate and threw for 3,877 yards with 20 touchdowns and five interceptions.

Bradford began this past season as the starter, but his own knee injury forced him to the sideline during a Week 2 win over the Bears. Enter Keenum, who was brilliant in helping lead the Vikings to an NFC North title and a berth in the NFC championship game.

"We have to figure out what is the best scenario for us with these three guys," Zimmer said Thursday. "We've won 40 games in the last four years because we've had a good football team, and I want to make sure with our team that we continue to build and improve, continually improve on defense... We're 30-4 when we score 21 points, so it's important that we don't lose the defensive part of the game because we keep the game close. So all these things become a factor."

Zimmer said bringing back one or all of the quarterbacks is "definitely" under consideration, but the Vikings are also vying for the services of Redskins quarterback Kirk Cousins.

"After we get here, after we get done looking at some of these young draftable guys, so we'll kind of sit down and figure out the direction we're going. But I feel very confident really in all three guys. I love all three of them. They're great people. I love how they work. They understand how we do things there as a team and the way we go about our business."

One issue with Bradford is his left knee and the ACL he's torn twice. Zimmer said his injury is "degenerative" and that throws a "monkey wrench in the whole thing."

"The process that we're going through right now with Sam, Teddy and Case, we're trying to determine exactly where these three guys are," Zimmer said. "Teddy is a guy that two years ago played fantastic. We thought he was a very ascending player. Obviously has a terrible knee injury, don't get to see him play in real, live action in basically two years. So there's the question with him.

"The question with Sam is two years ago he plays in 15 games, starts, plays well. This year, he plays in one game and gets hurt, so you go back and look at his track record.

"You go back with Case. He plays outstanding this year, and you go back and look at what he's done in the past and what he's done this year. Then really what we have to try to do is figure out what is the best scenario for us and understanding that trying to answer these questions on these three guys and try to go from there. So that's with (general manager Rick Spielman) and myself and (executive vice president of football operations Rob Brzezinski) and how it affects the salary cap and how it affects the rest of our football team."

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Zimmer knows the Vikings' decision about their quarterback is paramount to the team's success.

"It's important for myself and Rick and the organization we pick the right guy that can help continue to move forward," Zimmer said. "If we don't do that, then I'll probably get fired."