Knight damaged his right knee in a last-ditch tackle on Victor Anichebe during the first half of the 2-0 victory and was carried off on a stretcher. Coyle admitted the news could easily have been worse but Knight has formed an impressive partnership with Gary Cahill and his loss is a setback for the Trotters.
Coyle said: "Zat suffered ligament damage. It's not as severe as we first thought but certainly bad enough to keep him out for a period of time. There's severe swelling in the area so I think we'll know better in seven to 10 days when that goes down.
"But I think it's fair to say he'll be four to six weeks, which is a terrible loss, first and foremost for Zat, because he's played every game for me since I came to the football club over a year ago.
"He's been outstanding as a player and as a man and the sooner we get him back the better. It is a blow, there's no doubt about it, and we wish him a speedy recovery.''
Knight's injury opens the door for January signing David Wheater, who should now be given an extended run in the side, starting with Wednesday night's FA Cup Fourth Round replay at Wigan.
Meanwhile, young midfielder Mark Davies has signed a new four-and-a-half-year deal, keeping him at the club until the summer of 2015. The 22-year-old, who joined Wanderers from Wolves in January 2009, has emerged as a key player this season, making 25 appearances and scoring once.
Davies said: "I'm really pleased to have signed a new contract. The football club has been good to me since I moved here two years ago and I'm really settled in the area. The manager believes in me and I feel like I'm part of his plans.''
Coyle added on the club's official website: "Mark was already under contract but what this shows is that we are trying to build a football club for years to come. The nucleus is centred around young, exciting players and Mark certainly comes into that category. To have a player of his ilk commit his long-term future to the football club is a tremendous boost.''