Celtic boss Brendan Rodgers says Moussa Dembele could be out until early September with a hamstring injury.
The French striker, 21, missed the end of last season with a similar problem but featured in recent European ties.
Both he and suspended forward Leigh Griffiths, 26, sat out Wednesday's Champions League third qualifying round first leg against Rosenborg.
"It could be to the beginning of September," Rodgers said of Dembele's expected return.
"It looks like Moussa may well be out for the whole of the campaign, this European [qualifying] campaign so we have to adapt.
"Throughout the course of the season, unfortunately for those players with that speed and strength, there are always hamstring niggles.
"We saw it last year with Moussa. We saw it with James Forrest. We saw it with Scott Sinclair. We saw it with Patrick Roberts when Pat was here.
"We have to heal, work hard to get back as quick as he can, knowing that it's going to be a real long season for us."
France Under-21 international Dembele scored 32 goals in his debut season for Celtic after joining from Fulham last summer.
Earlier this week, Rodgers said the club had no plans to sell Dembele amid reported interest from Marseille.
Griffiths, who has scored 85 Celtic goals since 2014, is also nursing an injury.
"We're hoping that Leigh can come back at some point," Rodgers told BBC Scotland before Wednesday's game.
"Leigh's suspended and he's injured. He hasn't trained all week.
"We've always been based around the collective and we're looking at that collective to find solutions for us."
Asked if he may look to recruit another striker while the summer transfer remains open, Rodgers replied: "We'll see.
"We'll talk about it as a coaching staff and a club to see where we're at. We don't want to be to rash. We only ever want to bring in players that are going to enhance the quality."