"I'm not a dirty player," Mitchell said.
Smith was throwing on the run on the final play of the third quarter when Mitchell, who blitzed on the play, came in from behind. Mitchell said he lost his balance after feeling himself getting tripped and pushed. Linebacker Anthony Chickillo was behind Mitchell and put his hand on the safety's back as the two chased Smith.
Mitchell got a 15-yard penalty for roughing the passer, turning a third-and-8 into a 49-yard swing.
"The video shows what it shows. I wasn't trying to take Alex Smith's knees out," said Mitchell, whose Steelers won 19-13. "I went up to him and told him that...It's just one of those things you don't want to see in the football game."
Mitchell got a 15-yard penalty for roughing the passer.
Smith had words for Mitchell after the play and later told Kansas City media that the hit was "pretty late."
"I mean, when you get hit in the back of your knee like that, I didn't understand how that happened," Smith said. "Certainly guys falling, rushing the quarterback, I get it when it's happening from the front and guys are trying hard. That one to me just seemed so weird to get hit that low coming from behind."
Mitchell addressed the play with coach Mike Tomlin and general manager Kevin Colbert to make sure they understood his view of it. Mitchell added he's well aware of the NFL rule prohibiting defenders targeting the knees or lower legs of a quarterback.
"If you watch the tape, I'm even trying to turn my body as I'm falling," Mitchell said. "Alex is back pedaling into me, I wasn't even going in the direction of Alex. He's actually throwing it and fading away. There are a lot of things that go into the play that make it an accident you don't want to see."
Asked about the perception from fans on social media who reacted strongly to the play, Mitchell remains indifferent.
"I'm fresh out of cares about what people who don't know me say about me," he said. As for a fine, Mitchell said, "we'll see."