Lock, who suffered the injury on a second-down sack early in the third quarter of Monday night's preseason loss to the San Francisco 49ers, said following the game that he believed the injury to be a "little sprain," and X-rays taken at the stadium were negative for a fracture.
However, Lock underwent an MRI on Tuesday that confirmed a significant sprain. He is seeking a second medical opinion about the course of treatment.
Given the injury is to his throwing hand, Lock will miss practice time and is not expected to play in the remainder of the preseason. The Broncos play their final two preseason games Aug. 24. in Los Angeles and Aug. 29 against the Arizona Cardinals.
"I got out of the pocket and just ended up landing on that thumb,'' Lock said after Monday's game. "There are some videos out there, obviously, I am sure. It's just a little sprain. It should be all right ... I've jammed my thumb playing basketball 1,000 times. It kind of felt like a jammed thumb.''
Lock, whom the Broncos selected in the second round of April's draft, played 30 snaps against the 49ers and finished 7-of-12 passing for 40 yards with the sack when he was injured. Lock played one more snap after the sack and left the game with just over 13 minutes remaining in the third quarter.
After opening training camp as the No. 3 quarterback behind Joe Flacco and Kevin Hogan, Lock has been moved up to the No. 2 spot at times in practices over the past two weeks, and he was the second quarterback into the game against the Seahawks and 49ers.
Broncos coach Vic Fangio, who has made it clear the Broncos are going to be patient with Lock and that Lock has plenty of work to do in regard to his throwing mechanics and learning the offense, said Lock showed some improvement against the 49ers.
"I thought Drew showed some flashes of improvement, made some nice throws, moved around well, but I'm sure it wasn't perfect,'' Fangio said following Monday's game. "Again, he's improving and that's what we're looking for right now.''
Lock has completed 60.8 percent of his passes in the preseason with a touchdown and an interception.
Asked Monday night if he believed he had done enough to be Flacco's backup during the season, Lock said, "I'm going to let the coaches decide on that. I'm confident in myself that I can be a backup quarterback right now, but again it's not my decision to make that call.
"We still have two preseason games left to prove that and then get back and keep proving my statement for being the quarterback and the backup quarterback. I am confident that I can be that, it's just not up to me.''