"He was on the ground just kind of shaking his head," Thomas said.
Weeden sustained a concussion in the closing minutes of Sunday's 20-14 win over the Pittsburgh Steelers, an injury that brought Colt McCoy off the bench to close out only Cleveland's second win in 18 games against its biggest rival.
It was a twist for McCoy, who suffered a concussion late last season at Pittsburgh when he was leveled on a vicious hit by Steelers linebacker James Harrison.
Weeden appeared to bang his head on Thomas' leg as he fell awkwardly after throwing an incomplete pass with about five minutes left. The rookie was slow getting up and walked unsteadily to the sideline where he was met by medical personnel. Weeden was then escorted to Cleveland's locker room for further treatment and evaluation.
Thomas thought Weeden collided with Steelers linebacker Jason Worilds, who was applying pressure on the play.
"I thought his helmet hit another guy's helmet," Thomas said. "But anytime you're getting whipped down like that and getting that whiplash effect, it's not good on the noggin."
Browns coach Pat Shurmur said Weeden has a concussion but offered few other details. Shurmur spoke with his 29-year-old QB following the game.
"He's good," Shurmur said. "We had a good conversation. He's being treated for a concussion, but we'll see where that goes. Some guys come back quickly, some guys don't, so we'll just have to see where's that's at."
Weeden finished 17 of 26 for 158 yards with one touchdown and one interception before he was injured. His third pass was tipped and returned 53 yards for a touchdown by Steelers linebacker Lawrence Timmons.
McCoy did not attempt a pass as the Browns were intent on running out the clock in the final minutes. It was the first action this season for the third-year QB, who missed the final three games last season with his head injury and lost his starting job when Cleveland selected Weeden in the first round of this year's draft.
With the game still up for grabs, Thomas said McCoy brought a steadying presence into the huddle.
"It was good to get him out there," Thomas said. "He's a huge asset to this team, not only what he does in practice, but being able to be a backup who's ready, who's completely studied up as if he was the starter and completely ready and willing to take over the offense, to lead us just as well as Brandon.
"So having him in there with the confidence that he has and the games that he's played under his belt is huge."
If Weeden can't play next week at Oakland, McCoy would start and the Browns are confident he could step in and succeed.
"We've got Colt and Colt is going to do whatever he does," running back Trent Richardson said. "We're going to make sure Colt knows we believe in him. But I'm pretty sure Weeden's going to be back. Weeden's a warrior. He's been around for a while, including to his age. But he's a warrior.
"Weeden's one of the guys that don't want to get out. He didn't even want to go out then. They had to hide his helmet from him. So I'm pretty sure he'll be back. But if it comes down to it, we've got Colt and Colt makes smart decisions and Colt's been ready to play all year."