The Miami Dolphins now fear that quarterback Ryan Tannehill will need season-ending knee surgery; however, no decision on surgery has been made at this time, according to sources close to the situation.

As second and third opinions await, Tannehill still could decide to rest the knee for six to eight weeks and try to come back from the injury again, sources said.

But the Dolphins are growing increasingly concerned that surgery will be the most viable and likely option as they and their starting quarterback sift through their choices.

"He's done, I think," one source predicted Thursday night, referring to Tannehill's hopes of returning this season.

Tannehill must decide whether he wants to wait at least six weeks to let the knee heal and take another shot at playing this year or go ahead and have the surgery, which would end his season.

The Dolphins recognize they might have to step in and recommend the surgery that Tannehill has avoided since he initially injured the knee in December against the Arizona Cardinals.

One source said Thursday that, because Tannehill did not repair his partially torn ACL during the offseason, his knee was "a ticking time bomb that was going to go off at any time."

It happened Thursday, on a noncontact play, when Tannehill crumbled to the ground while scrambling in practice.

Tannehill underwent an MRI, and the Dolphins know they will be without him for at least a significant amount of time.

The Dolphins have a competent backup in Matt Moore, who took over first-team reps for the remainder of Thursday's practice after Tannehill was hurt. But Moore was prone to some big hits last season that could make him vulnerable to injury as well.

Sources said Jay Cutler would be willing to delay his broadcast career at Fox to play for Dolphins coach Adam Gase, who helped Cutler revitalize his career when he was the quarterback's offensive coordinator with the Chicago Bears in 2015. But Cutler plans to consider such a move only if he gets the chance to be the full-time starter -- and get starter money -- for the entire season. Cutler views this as the only practical quarterback job he would consider at this point, sources said.

Cutler was the one with the initial interest in playing for Gase, sources said, and the Dolphins have not yet gone down that road.

In addition, sources told ESPN's Josina Anderson that Colin Kaepernick's name has been brought up within the Dolphins' front office as potential insurance at the position. Miami owner Steve Ross has shown in recent years to be open-minded regarding social issues, defending the choices of some Dolphins players last season when they joined Kaepernick by taking a knee during the national anthem. Gase, too, is known as a player's coach willing to maintain an open mind on off-the-field beliefs.

Tannehill is coming off his best, most efficient season in 2016, having gone 8-5 as a starter with a career-best 93.5 passer rating before he suffered the season-ending injury Dec. 11. Moore went 2-2 while filling in for Tannehill.

"I just saw him kind of buckle and go down. I didn't see any details," offensive coordinator Clyde Christensen said of Tannehill's injury Thursday. "There was no contact on the play. That was all I really saw."

Said tight end MarQueis Gray: "I saw it, but I have no idea what the outcome will be. We're just praying for him. We don't know what's going to happen."

This is another key injury for the Dolphins in the first week of training camp.

Pro Bowl running back Jay Ajayi suffered a concussion during Monday's practice and is in the concussion protocol. Starting left guard Ted Larsen suffered a biceps injury Thursday and is expected to miss several weeks of the regular season, according to multiple reports.

"The initial reaction is just be the next man up," Christensen said. "You gotta keep going. You gotta keep guys going, and you can't let the air go out of practice. That's football. [Injuries] are going to happen somewhere in this season. It's going to happen somewhere in this game."

Information from ESPN's James Walker was used in this report.