Rosicky suffers hamstring setback

Rosicky has been involved in Arsenal's pre-season friendlies

Arsenal midfielder Tomas Rosicky, who has been out for 18 months due to injury, has hurt his hamstring in training.

The 28-year-old's last appearance was in an FA Cup tie against Newcastle in January 2008, and since then he has struggled with hamstring and knee problems.

Arsenal boss Arsene Wenger described the injury as a setback but did not say whether the Czech international would miss the start of the new season.

Rosicky injured the hamstring in training on Wednesday. Arsenal's policy is to send players for a scan if they pick up injuries.

"Six weeks ago I did not know how strong Eduardo (Da Silva) would come back, or how strong Rosicky would come back, despite the setback he has had," said Wenger, whose team start the Premier League season at Everton on 15 August.

606: DEBATE Give your reaction to Rosicky's latest injury setback

Meanwhile, Wenger believes Manchester City and Aston Villa will challenge the Premier League's "big four" of Manchester United, Liverpool, Chelsea and Arsenal.

"City look like they have made some great signings and have a high-quality squad, but it can take teams three months or more to find the right balance and to click," Wenger said.

"But I think City will be there and be strong and remember that Aston Villa were challenging for the title (last season) until they faded in the last 12 games.

"When I arrived, Newcastle were there, at the top, and then it was Leeds," he added. "So I think the top four is not something that is set for ever."

Rosicky looked sharp - Wenger

The Frenchman also stated he was interested in adding one defender and one forward to his squad, at the right price, before the transfer window closes at the end of August.

"Don't forget that most of the business is done between 26 August and 31 August, at midnight," said Wenger, who was speaking at the launch of Arsenal's charity partnership with Great Ormond Street children's hospital in London.