Bolton defender Samuel Ricketts has been ruled out for the remainder of the season with a ruptured Achilles tendon.
Ricketts is expected to be out for around six months and is due to undergo an operation on Friday on the injury picked up in the midweek FA Cup win at Wigan.
Coyle said: "Unfortunately Sam's ruptured his Achilles and he'll be operated on today but the likelihood is he'll be missing for about six months, which is an incredible blow first and foremost to Sam, because he's been outstanding for us.
"Our immediate thoughts are with him for a speedy recovery and we'll just have to get on with it. It was totally innocuous and it's a real shame because he's a terrific lad as well as a fantastic footballer.''
Wigan's pitch was relaid following Wednesday's cup tie but Coyle denied the state of the playing surface could have been a factor in Ricketts' injury.
He said: "There's no blame to the pitch. Roberto [Martinez, Wigan manager] will tell you it's not a great pitch, certainly not conducive to the way Wigan want to play, but there's absolutely no blame apportioned to the pitch with regards to the injury.''
The Bolton boss also confirmed Knight is likely to be out for six weeks after further assessment of his knee problem. Coyle added: "He's making progress but we still expect Zat to be missing for six weeks, which is not ideal but that's the nature of football with the speed the game's played at at this level. It's a crying shame for both lads because they've been in terrific form.''