Rookie coach Mike McCoy, who's loath to discuss injuries, said the concussion was diagnosed after the game. The first word of that diagnosis came when the team announced its injury report after practice Wednesday.
It's not known whether Hardwick will be able to play Sunday in the divisional round game at AFC West rival Denver, the AFC's No 1 seed. He must pass certain tests in order to be cleared to play.
Rookie right tackle D.J. Fluker also sat out practice with an ankle injury, as did running back Ryan Mathews.
Rich Ohrnberger replaced Hardwick on Sunday and played well.
Quarterback Philip Rivers said he'd feel comfortable behind Ohrnberger again if Hardwick can't play Sunday.
''I don't want to ever speak like I don't appreciate Rich and what he did, and if he has to do it again we're confident he will,'' Rivers said. ''A guy like Nick, it's been a lot of years. We've spent a lot of time together, both in that meeting room, off the field and out here on this practice field. So from a communication standpoint, that's the biggest thing.
''Obviously Nick's a great player but he knows sometimes what I'm saying even if I don't say it right. Rich and I have been in 50-60 snaps together, maybe a little more than that, where Nick and I, there's no telling how many. That would be the biggest thing with the noise and him understanding what I was trying to get to or what we're doing.
''You never want to see Nick go down, but if he were not able to go, last week got us a week of preparation with Rich stepping up in there so I'll definitely feel more comfortable now if that's the case.''
Said Ohrnberger: ''I'm just happy it all worked out. Phil's the consummate professional and having a guy with that sort of leadership behind you helps you out a lot.''
Will he be the starter Sunday at Denver?
''As far as injuries go, you have to ask Coach McCoy about anything regarding that,'' he said. ''All I know is that I'm going to be as prepared for the game as I possibly can be.''
Mathews, plagued by an injured left ankle for several weeks, came out of the victory at Cincinnati in the third quarter after gaining 52 yards on 13 carries. He continues to wear a walking boot to protect the ankle.
''My mindset's on playing. I've got to stay as mentally ready as I can because I'm not going to get snaps in practice,'' said Mathews, who ran for 127 yards and a touchdown in San Diego's 27-20 upset victory in Denver on Dec. 12.
Asked if he expects Mathews to play, McCoy said: ''We'll play the healthiest 46 guys we've got.''
Wide receiver Eddie Royal (toe) also didn't practice. Limited were right guard Jeromey Clary (shoulder), defensive tackle Sean Lissemore (shoulder) and safety Eric Weddle (hamstring).
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