"He couldn't protect himself anymore," Mitchell said.
Placed on injured reserve Tuesday, Smith had complained about his shoulder throughout the two previous weeks, leading Mitchell to believe Smith had begun feeling the effects of the torn rotator cuff earlier in the season, because "Aaron is not a complainer. That's the last thing he'll ever do. So when he did tell us, you figure it had to really be bothering him."
In spite of the injury, Mitchell said Smith was playing at a high level up until the Detroit game.
"He graded out very high in the previous two games," said Mitchell, who expects Smith to return next season and resume playing at his Pro Bowl-caliber level.
Smith's primary replacement Sunday against Cleveland will be 35-year-old Travis Kirschke, who played in a season-high 31 snaps (42 percent of the defensive total) in Detroit. Kirschke will be expected to push those numbers for as long as he's effective, with Nick Eason the first replacement and rookie Ziggy Hood receiving spot duty.
Casey Hampton, who's played in 48 percent of the defensive snaps this season, will receive more work in the pass-downs defenses, which in turn will mean more work for Chris Hoke at nose tackle in the base. Hoke has played only 19 snaps (6 percent) this season.
The iron man of the Steelers' defensive line has been Brett Keisel. The right defensive end has played 283 snaps, or 90 percent of the defensive total. Smith is second among the ends with 250 snaps, followed by Kirschke with 67 and Hood with 23. Hood's 16 snaps in Detroit is his season-high.
"He's doing a great job," defensive coordinator Dick LeBeau said of Hood. "We drafted him because of his work ethic and his production in college, and we have seen nothing to deter us from thinking we've got us a really potentially very solid, solid football player, and he's working hard to get there. I see him getting better every day. It's a tough loss for us, losing Aaron, but it'll give Ziggy more snaps and in that respect will be a good thing for that young man."
LeBeau was asked why the team isn't planning to give Hood a larger portion of the replacement snaps in order to hasten his development.
"Woody Hayes once said, ‘You lose two football games for every freshman you've got in your lineup,"' LeBeau said. "I think that's a valid statement. But we'll play all of our players who can be productive for us. We don't care what year they're in. We intend to have Ziggy in the game. He played quite a bit in Detroit. His playing time has been increasing as we go along, and we'll see. We'll play the guys who are giving us the most production. That's the way this business is."
Another player who could be bucking for more playing time is strong safety Troy Polamalu, who's returning from a 4-game absence due to an MCL sprain. He appeared to practice at half speed Thursday and seemed to have difficulty making cuts. He practiced with a brace, which he'll use in the game – if he plays.
"The head coach, that comes in his area," said LeBeau when asked about Polamalu's playing time. "I feel good about Troy, and then Mike (Tomlin) will address those issues whether Troy's up, down, how long he'll play, or how many snaps."
PARKER'S FULL WORKLOAD
Willie Parker took most of the first-team base reps Thursday because Rashard Mendenhall was home with the flu. Mendenhall's expected to return Friday, and then the coaching staff will decide on the division of labor for the running backs on Sunday.
"We'll probably go up till the last minute and see what's up," said offensive coordinator Bruce Arians.
Frank "The Tank" Summers showed up Thursday at the South Side facility and the rookie runner said he'll show up every day because "I have nothing else to do, no friends or family here."
Summers was placed on injured reserve Sept. 29, and soon thereafter underwent surgery on a herniated disk in his back, which he said requires a three to four-month rehabilitation.