Everton midfielder Mikel Arteta has suffered a grade-two tear hamstring tear.
Arteta had to be withdrawn after just seven minutes of his side's 1-1 draw against Birmingham on Wednesday night and David Moyes later said he feared the injury could be significant.
"It looks like a hamstring injury," he said on the club's official website. "I'm guessing he'll be out for a few weeks."
Having undergone a scan on Thursday, Everton have now said he has suffered a grade-two tear, although a club statement said "it is not yet known how long the Spanish midfielder will be out for".
Steven Gerrard was sidelined for four weeks with a similar injury earlier this season.