Ranieri said that the 29-year-old had an operation on Thursday.
"The hands for keepers, like the hands for boxers, are very, very important," the Italian told a news conference on Friday.
"I don't put a deadline because every man is different. Maybe two weeks, maybe three, maybe five, six, seven. I don't know. Kasper wants to play on Sunday. It's normal."
Schmeichel picked up the injury in the first half in Copenhagen, but played the whole match.
"Only in the night he felt something," the manager added. "It's unbelievable. Maybe (because of) the adrenaline, the concentration, he didn't feel anything (on the pitch)."
Zieler, who was signed from Bundesliga side Hannover 96 in the close season, will start in goal for 11th-placed Leicester when they host 16th-placed West Bromwich Albion on Sunday.
Ranieri said he had no qualms about the 27-year-old making his fourth league appearance of the season and expected a tough encounter against Tony Pulis's side, who drew 2-2 at the King Power Stadium last season.
"I’m very confident with Zieler. Of course Kasper was in great condition but Zieler gives me confidence," Ranieri said.
"I expect the same match as last season. A very tough match. West Brom defend and counter very well.
"We want to keep going and improve in the table... We're getting better but it's not only the defenders. All the team are getting better with the fighting spirit."
Close-season signing Islam Slimani, who has been struggling with a groin injury, will return to the squad on Sunday, while Nampalys Mendy (ankle) is improving, but not yet ready to feature.
(Reporting by Simon Jennings in Bengaluru; Editing by Toby Davis)
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