The Foxes No.1, who has started 14 of City’s 15 Barclays Premier League games this season, will undergo surgery on Friday.
Summer signing Ben Hamer, in his only Premier League appearance for the Foxes back in September, is set to make his home league debut against Manchester City on Saturday.
“Kasper will be having an operation tomorrow on a metatarsal that he’s unfortunately broken this morning,” said Pearson. “There was no contact. It’s very unfortunate. Clearly the lad is distressed about it and he’ll be a big miss for us. What you have to do is deal with it – we can’t change it.
“I’ve got an idea [of timescale] but the best thing to do is give an update on Saturday. Hopefully he’ll have the operation tomorrow.
“An optimistic view would be four to six weeks but it could be longer. We don’t know what the surgeon will find when he deals with the situation.
“Clearly when you miss your number one goalkeeper with a couple of days to go that’s not particularly helpful. We’ll deal with it because that’s how it is."
Meanwhile, Leonardo Ulloa has shrugged off a knock that caused him to be substituted at half-time in the .
Pearson added: “He’s okay. As I said last week, it was heavy contact but he’s fine. We’ll have a pretty fit squad. Dean Hammond will be back in contention which is good as well.
“Matt Upson will probably need a bit longer for him to come through a couple of games. It would be nice to have his experience available. The welfare of the players is important. We want him to stay available.
“We’ve had similar situations before where, when players get very close to being available, it can be dangerous to expose them to that next step. It’s difficult to be patient in times like this but it’s still important to do things the right way.
“Hopefully when he is available for the first team to be selected he’ll make an important impact for us. I won’t consider him for this weekend.”