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.
Luke Shaw
PSV defender Moreno was not booked for the challenge on Shaw
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. The Welshman tweeted: "Fingers crossed for Luke Shaw. Get well soon."
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.

'A very bad tackle with two legs'

Hector Moreno
Moreno went over to see Shaw as he was taken off the pitch
Mexican centre-back Moreno escaped censure for the challenge on Shaw and Van Gaal was unhappy 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."

'A devastating blow'

Luke Shaw
Shaw wrote a message on Twitter with an hour of the injury being sustained
Analysis from chief football writer Phil McNulty: The serious injury sustained by Luke Shaw is not only a devastating blow to the 20-year-old, who had made such an outstanding start to this season, but also to Manchester United and manager Louis van Gaal.
Before the start of the campaign Van Gaal said: "I think that this will be the season of Luke Shaw - that I believe." Now those hopes have been cut down by an injury that looked so bad that the defender will be sidelined for months.
In a somewhat stodgy start to the new season, Shaw had shone as he looked leaner, sharper and more settled than at any time during a mixed first year at Old Trafford.
The sight of the left-back being stretchered off against PSV Eindhoven also capped a poor day for England manager Roy Hodgson after Arsenal's Jack Wilshere was ruled out for three months as he requires surgery on a hairline fracture of his left leg.

What could Shaw miss?

194 days: England play Germany in a friendly (26 March, 2016)
212 days: Henrik Larsson's recovery time after a double leg break
218 days: Bryan Oviedo's recovery time after a double leg break
243 days: Final day of the Premier League season (15 May, 2016)
250 days: FA Cup final (21 May, 2016)
252 days: Aaron Ramsey's recovery time after a double leg break
269 days: Euro 2016 begins in France (10 June, 2016)
296 days: Eduardo's recovery time after a double leg break
299 days: Euro 2016 final (10 July, 2016)

Man United team-mates react

Shaw, who made 20 United appearances in an injury-interrupted first season after joining from Southampton in a £31m deal, was inundated with messages of support on social media from his team-mates.
Goalkeeper David De Gea said "defeat hurts, but Luke Shaw's injury hurts even more".
Defender Chris Smalling said his "thoughts and prayers" were with his team-mate and midfielder Ander Herrera wants Shaw to "come back soon, my friend".