The Chile forward injured his hamstring in training on Thursday and will miss Saturday’s trip to Southampton and Wednesday’s home clash with former club Arsenal.
“Alexis sustained a hamstring injury in training on Thursday which requires further investigation to confirm severity,” a statement from the club said.
It marks the latest of a long line of setbacks for the 29-year-old since his move from Arenal in January.
He has started only seven matches this season and was left out of the squad for the Champions League win over Young Boys.=
Sanchez is likely to be out for between two and four weeks and might have now played his last game for the club if reports that say he is set to leave come to pass.
It is claimed he has told friends he is ready to quit Old Trafford less than a year after making the lucrative switch.
Sanchez earns a reported £500,000 per week – making him the Premier League’s top earner – but is said to be unhappy after being frozen out of the first team.
Paris Saint-Germain remains the most likely option if he is to move and reports claim he has his heart set on a move to the French giants.
However, manager Jose Mourinho says he expects him to stay beyond the January transfer window.
“He has never told me that he wants to leave. He never told me that he’s not happy to stay,” Mourinho said.
“He is playing like the team. He is improving, like the team is improving, when he plays he is giving us his personality.
“His desire is to play and to produce for the team and [he is] trying to adapt to the way we play and vice versa.
“Can he do better? We all can do better. I can do better, he can do better, all the players can do better.”
Sanchez has scored just four goals in 30 games in all competitions for Mourinho’s struggling United side.
They are currently seventh in the Premier League table, seven points off the Champions League places and a whooping 14 behind leaders Manchester City.