Sunderland boss Gus Poyet says defender Anthony Reveillere is facing three to four weeks on the sidelines.
Reveillere, 35, injured a muscle between his Achilles and calf during the warm-up for Sunderland's derby clash against Newcastle on Sunday.
Poyet said the club had initially feared the problem was more serious, but the former France international will still be out for up to a month.
"It's a muscle injury," Poyet told a news conference. "We thought it was more because of the way it was translated...