Defender Richards was carried off on a stretcher after pulling up less than three minutes into the 4-0 thrashing of Denmark in Viborg.
The injury-plagued 22-year-old claimed afterwards he would be out for four weeks, which would rule him out of the April 16 Manchester derby at Wembley, but head coach Stuart Pearce insisted no medical prognosis had been given.
"He's got a hamstring problem and he's diagnosed himself as being out for four weeks - that's Dr Richards for you," Pearce said.
"That could be four days, though, as that's him diagnosing himself!
"Let's hope for Manchester City's sake and the preparation in the summer that it's not too long."