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. I don't like to say 'x days' out. Let's wait. It's a muscle injury."
Rojo's injury is badly timed as United have lost defender Eric Bailly to the African Nations Cup with Ivory Coast. They face a busy schedule, fighting on four fronts, but Mourinho said on Friday that he does not plan to recruit another defender.
Jones stepped in and performed well, alongside Chris Smalling, as Mourinho's men kept a clean sheet against Reading, who are managed by former United defender Jaap Stam.
Discussing his team's display, Mourinho said: "It was very professional. I could feel it from the first minute. The team was intense. The team was dynamic.
"I didn't learn [from the perfomance]. It just confirmed what I already know. They trained well, have a big desire to play and are good friends with each other."