Lukas Podolski will be out of action for up to 10 weeks after his hamstring injury proved worse than initially feared.
The Arsenal forward was stretchered off shortly after half-time during Tuesday's UEFA Champions League victory over Fenerbahce.
Arsene Wenger estimated a three-week absence for the German in the immediate aftermath of the 2-0 win, but further investigation has revealed a more significant injury.
"It is bad news about Lukas Podolski," Wenger said. "He will be out for eight to 10 weeks.
"His hamstring is much more damaged than we thought at the start. It is very disappointing and we have paid a very heavy price for that win. We have to be patient now."
The news is likely to intensify calls among Arsenal supporters for new recruits to be brought in before the transfer window closes on Monday.
Mikel Arteta, Alex Oxlade-Chamberlain, Thomas Vermaelen and Abou Diaby are all out injured, and Wenger is still to spend a penny in transfer fees this summer despite the "escalation in financial firepower" reported by chief executive Ivan Gazidis back in June.
Yaya Sanogo and Mathieu Flamini are the only arrivals to date, and both were free agents.
Wenger has admitted defeat in his attempts to buy Luis Suarez from Liverpool while Real Madrid's Karim Benzema, Mesut Ozil and Angel Di Maria - all reported targets - want to stay at the Bernabeu.
There is, at least, better news on the fitness of Aaron Ramsey and Jack Wilshere ahead of Sunday's north London derby with Tottenham.
Both midfielders finished the Fenerbahce game injured but, according to Wenger, their "bumps and bruises" will not keep them out against Spurs.