Full-back Luke Shaw suffered a double fracture of his right leg in Manchester United's Champions League loss at PSV Eindhoven.
Shaw, 20, will have an operation on his return to Manchester, the club said.
He received oxygen while being treated on the pitch for 10 minutes after Hector Moreno's tackle.
"It is awful. It was a very bad tackle. He had the oxygen mask on and was crying in the dressing room," said manager Louis van Gaal.
The left-back started each of United's eight games this season, as well as playing in England's European Championship qualifiers against San Marino and Switzerland.
Yet with just nine months to go until Euro 2016, Shaw has suffered a broken tibia and fibula.
When Arsenal midfielder Aaron Ramsey suffered a similar injury against Stoke in 2010, he was ruled out for nine months.
Less than an hour after suffering the injury Shaw wrote on Twitter: "Thank you everyone for your messages, words can't describe how gutted I am, my road to recovery starts now, I will come back stronger."
Former PSV man Memphis Depay scored the opener for the visitors in Eindhoven, before Moreno headed in an equaliser on the stroke of half-time.
Luciano Narsingh then nodded home in the second half to secure a 2-1 victory.
Shaw as he was taken off the pitch
Mexican centre-back Moreno was not booked for the challenge on Shaw and Van Gaal was not happy that the referee took no action.
"A boy who came to Manchester United at 18 years old, it was very difficult, and the next season he plays fantastic and then that happens like this," said the Dutchman.
"When I say it is a red card and penalty, you will say I am a bad loser. Every word I say is taken in the wrong way. It was in the 18-yard box and it was very bad tackle with two legs.
"I'm not a doctor so I cannot say, but when you have a double fracture it is four to six months. But you can never say that in advance.
"He won't play in the group stage, I don't think. I hope he will play again this season."
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