Aston Villa midfielder Stiliyan Petrov will be out of action for at least two weeks with a hamstring problem. The Bulgarian international suffered the setback during the first half of Saturday's home Barclays Premier League clash with Arsenal. The former Celtic player will definitely miss Saturday's home game with Portsmouth and the away meeting with Sunderland the following week. A scan on Monday revealed the problem and the timing of the injury could not be worse as he has been showing his best form since arriving at Villa 16 months ago. Villa manager Martin O'Neill said: 'We have had the scans back and Stiliyan has definitely got something there that will require a couple of weeks out. 'I can't be absolutely sure of the time span but he certainly looks as if he will be missing for a fortnight and that is a real shame for him. 'Since he came back into the side, Stiliyan has been doing very well and has been close to getting back to his best form. 'He has only shown in flashes what I know he is capable of from his time at Celtic. Things haven't gone as well as he would have liked but recent signs have been encouraging. 'I think his performance against Birmingham in the recent derby was big for him. That was a turning point, he had kept that going and his confidence was growing.' Petrov's absence means that Craig Gardner could keep his place against Pompey despite the return of Nigel Reo-Coker from suspension. Gardner scored his third goal of the campaign against Arsenal and has impressed when filling in on the right flank.