Manchester United manager Louis van Gaal has confirmed that striker Wayne Rooney has suffered a knee injury and could be ruled out for weeks.
The Englishman joins a long list of key players not available for the Dutchman as the Red Devils prepare to face FC Midtjylland away from home.
Matteo Darmian, Marouane Fellaini and Cameron Borthwick-Jackson also stayed behind in Manchester.
"We have 13 players out and Wayne is one of them," van Gaal told MUTV.
"You can never say [how long he could be out for]. He has a knee problem so we have to wait and see.
"Certainly in 2016, he has scored goals and that's why we had a good run in matches until Sunderland, but we have to cope with that.
"Everybody can be injured and also the captain can be injured.
"Wayne [is a player] who wants to go until the end and if he is feeling pain, he doesn't want to go away [off the field], and certainly when you are drawing a match, he wants to win. That's a fantastic attitude but also it's bad for his body.
"Darmian has a dislocated shoulder. It's not the heaviest type so we are lucky, but he is injured and out.
"Borthwick-Jackson is ill and cannot play either, so we have 13 players injured or not capable of playing.
"I have never had that amount of players injured [before]. When you realise that they are only contact injuries - only Marouane Fellaini maybe has a muscle problem, the rest have contact injuries - it's unbelievable that we are very unlucky."