"This was one of the great mistakes that I missed in my coaching at USC, was how good of a player he was," Carroll told reporters Thursday as the Seahawks prepare to host the Packers in the NFC championship game.
Matthews walked on at USC in 2004 and did not start until the fourth game of his senior season.
Carroll recalled a scene from Matthews’ junior season when a reporter asked about his play.
"I said, 'He may be one of the best special teamers that ever played at USC,'" Carroll said. "I couldn’t really validate that, but I was blowing him up because he was a great kid."
Matthews got a chance to validate himself when Carroll inserted him at defensive end in the third game of the 2008 season against Oregon State.
"We had some problems and we put him in the game and said, 'Well, we know Clay will do the right thing,'" Carroll said.
Matthews made five tackles, blocked a field-goal attempt and started the final 10 games.
"He’s been a great player since the time he was given the opportunity to show it," Carroll said.
This season, Matthews has 11 sacks, two forced fumbles and an interception.
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