Thomas tweeted Monday that the injury was a torn triceps tendon. On a conference call, he said he will probably have surgery Tuesday and that a full recovery may take six to nine months.
He was less definitive on the future of his NFL career.
"Am I done playing? I don't know," Thomas said. "I think that's a decision that's best talked about in the offseason."
Thomas, 32, noted that he still has one year remaining on his contract.
"That decision about do I continue to play or do I retire I think is something I've always left up to the offseason," he said. "I think it takes some time to get away from football."
He had not missed a snap for the Browns since he was taken third overall in the 2007 draft, a streak of 10,363 consecutive plays that the team said is unprecedented in NFL history. He is also one of five players in NFL history to make the Pro Bowl in each of their first 10 seasons in the league, joining Mel Renfro, Merlin Olsen, Barry Sanders and Lawrence Taylor.
Thomas was officially placed on injured reserve Monday.
"As soon as it happened I kind of knew what happened," Thomas said. "I could feel the tendon detaching from my elbow and get rolled up into my mid-triceps. I really had no function in that arm. I knew right away that I wasn't going to be able to go back out there even if I wanted to."
Thomas' coach and teammates commented on the injury Monday, including quarterback DeShone Kizer calling the situation "heartbreaking."
"He is a guy that keeps us all going and moving forward," coach Hue Jackson said. "So we owe him more than what we've given him thus far, that's for sure."
"I kind of thought for a second that he was unbreakable, and it was like nothing could go wrong with him," said guard Joel Bitonio, who before the injury had lined up next to Thomas every play he has been with the Browns. "To see him go down, you kind of realized he's a mortal among us."
Thomas was able to keep his sense of humor, even with the injury, as his Monday tweet shows:
The Geneva convention on torture (confinement, stress positions) was just violated when they wedged me into that MRI tube for 2 hrs. Yikes!
— Joe Thomas (@joethomas73) October 23, 2017
"It's definitely surreal," Thomas said. "Obviously you don't ever expect to get injured, but as a football player, you always understand the reality of the sport that we play is that at any moment it can be taken away from you and you can be sitting on the bench rehabbing an injury. I think in the back of your mind you're always prepared for it, but you're not always ready for it."