In hindsight, they really did. They knew what they had on the OL and presumably knew where Mateer was in terms of recovery. I didn't watch them before the RRS so I don't know what, if anything, he was lacking. He made some really impressive short to intermediate throws but seemed to be a non factor in the vertical game. I don't know if that's typical or if the thumb was impacting his ability to throw downfield. He also seemeed to just be overwhelmed by the moment and the pressure Texas was putting him under. Seemed to me that to the extent Mateer got them in trouble it was his mental game, not his thumb.
They had to know, like a lot of people expected, that it was going to be tough sledding on offense if they couldn't run the ball even wih Mateer. Their defense (and/or the Texas offense) was always going to have to win the game for them. Knowing that, if Mateer is limited, I don't see why you start him. You go out and lose a competitive game against Texas with Hawkins and it gets basically written off in playoff converssations (and in your fanbase, and re: Mateer's heisman campaign). You go out and win because you don't throw like eight interceptable balls, all the better. Instead you rush Mateer back, let this savior narrative take hold, and watch it flop in a game you were never going to win if your defense got worked like that.
Now the book is fully written on how to beat Oklahoma and they're staring down a brutal schedule with a crisis of confidence in leadership. What a self own.