Manchester United defender Marcos Rojo has suffered a muscle injury and the club are waiting to find out the extent of the damage.
Rojo, 26, started for United in the FA Cup third round win against Reading at Old Trafford on Saturday but the Argentina defender lasted just 19 minutes when he was substituted for Phil Jones.
United were already leading 2-0 at the time, through strikes from Wayne Rooney and Anthony Martial, and Marcus Rashford went on to score twice in a 4-0 victory.
When asked how Rojo is, Mourinho replied in his postmatch news conference: "I don't know.