DAVIE, Fla. -- Miami Dolphins quarterback Ryan Tannehill reinjured his left knee in practice Thursday when it buckled as he scrambled and fell without being hit.
Tannehill walked slowly off the field accompanied by trainers. He underwent an MRI, but results weren't definitive regarding the extent of the injury, and further assessment was expected Friday.
According to Adam Schefter and Jeff Darlington of ESPN.com, the Dolphins believe Tannehill will need season-ending surgery.
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"You see your quarterback go down, you think the worst," tight end MarQueis Gray said. "He's in all of our prayers, and we hope he's back on the field."
Tannehill missed last season's final four games, including a playoff loss at Pittsburgh, after spraining two ligaments in the same knee. He decided against surgery and took part in all offseason drills.
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He was wearing a brace when his knee gave out near the right sideline while running full speed to escape tackle Ndamukong Suh. Tannehill stayed on the ground at least 15 seconds surrounded by teammates before rising and leaving the field.
"I saw him buckle and go down. There was no contact on the play," offensive coordinator Clyde Christensen said. "The initial reaction is, `OK, next man up.' That's football. It's going to happen somewhere in this season. The game doesn't stop. You go on to the next play. And then, obviously, my personal thing is, `Boy, I sure hope it isn't serious.'"
The injury occurred during the seventh practice of training camp.
Matt Moore, an 11th-year veteran, went 2-2 as a starter replacing Tannehill last year and remains the backup.
"There's a reason we have Matt Moore here," Christensen said. "We hope he never has to play, and if he does, he'll be ready."
The Dolphins were already without 1,000-yard rusher Jay Ajayi, sidelined since Monday by a concussion. Their top lineman, center Mike Pouncey, has seen extremely limited activity as he returns from hip surgery.
The Dolphins spent several weeks after last season assessing the extent of Tannehill's ACL and MCL sprains before ruling out surgery, which could have jeopardized his availability for the start of this season. Tannehill rehabilitated the partially torn ligaments, underwent stem cell treatments and has been saying since May he felt 100 percent.
"Dealing with an injury like that for the first time, it was a tough process for me," Tannehill said last week after the first practice of training camp. "But the building, everyone around me, really supported me throughout that whole process. I'm glad to be back out here and feeling really healthy again."
If Tannehill's injury leaves the Dolphins in the market to sign another quarterback for depth, candidates include Colin Kaepernick, who parted ways with the San Francisco 49ers in March and remains unsigned. His decision last season to kneel during the national anthem to protest police brutality prompted national debate.
Last week, Dolphins owner Stephen Ross said he hoped Kaepernick wasn't being punished by teams for his politics.
"I know that a lot has been written about it, but you know owners or coaches - they'll do anything it takes to win," Ross said. "And if they think he can help them win, I would hope they would sign him."
Pro Football Talk contributed to this report