Arsene Wenger's side looks like it could be struggling in midfield once again this season.
The Gunners are already without Jack Wilshere after the England international once again damaged his ankle, and they now face the prospect of losing another central midfielder early on in the season.
The 34-year-old Czech Republic international found himself out of the first team for much of last year, playing just 15 games in the Premier League even though he featured in 24 games overall.
But he was a vital option in midfield during a campaign when the likes of Wilshere and Aaron Ramsey suffered injuries that ruled them out for substantial periods, and it looks like that injury jinx has hit again in the middle of the park, meaning Arsene Wenger could be short of midfield players that could offer an attacking role from the centre of the team.
His agent told Czech publication iDNES that the former Borussia Dortmund man has undergone surgery on his knee and could miss two months of the season while he recovers, with the Czech captain struggling to be fit for international matches against Turkey and the Netherlands in October.
"Tomas last Tuesday had arthroscopic knee surgery. The recovery period is estimated at four to eight weeks," Pavel Paska is quoted as telling iDNES.