The Bulls announced the injury Monday, saying that Markkanen has a sprained right acromioclavicular joint. He injured the joint at the top of the shoulder during Friday's loss to the Orlando Magic.
Bulls coach Billy Donovan described the injury, which happened when Markkanen was fouled by forward Gary Clark, as a "bit freakish," with Markkanen "driving to the basket, being physical."
Markkanen, 23, is averaging a career-best 19.1 points and 6.1 rebounds on 51.4% shooting in his fourth season. The 7-footer missed seven games in the season due to the NBA's health and safety protocols.
"We've just got to pick up where we fell off. Obviously, we are missing two real big keys to our offense and certain things like that," Bulls forward Daniel Gafford said after Monday's shootaround. "But Lauri and Wendell, they're pretty good guys and they already know that stuff happens.
"Guys that are coming in behind them, we've just got to step up and take their spots for the time being until they get back and they're fully recovered."
Information from The Associated Press was used in this report.