Paul Pogba was removed from Manchester United's 1-0 victory against Rostov on Thursday with a muscle injury.
Pogba, United's world-record signing last summer, was forced off in the opening moments of the second half, walking directly to the training room after being subbed out for Marouane Fellaini.
United defender Daley Blind also came off after taking a knock to the head. Phil Jones took his place in the 64th minute.
United boss Jose Mourinho confirmed Pogba will definitely miss Sunday's game against Middlesbrough as well as France's internationals against Luxembourg and Spain.
"I don't know for sure how long but for sure no Middlesbrough and no national team," Mourinho said after the match.
Mourinho also suggested fatigue from United's packed fixture schedule played a part in the injuries.
The France international has struggled to find his best form at Old Trafford and has recently faced heavy criticism for his performances, particularly after United's 1-0 defeat to Chelsea in Monday's FA Cup quarterfinal.
Pogba has scored seven goals and provided five assists in 41 appearances over all competitions for United this season.