Manchester United left back Luke Shaw suffered a double fracture of his right leg in the early stages of their Champions League Group B defeat at PSV Eindhoven on Tuesday.
"It is a double fracture. He will go to Manchester for surgery. It is very bad and sad for him, but also for our team," said United manager Louis van Gaal.
Shaw, 20, was injured under a challenge from defender Hector Moreno as he stormed into the penalty area at the end of a run towards goal in the 15th minute.
The game was stopped for eight minutes while medical staff attended to the England international, who was then carried off on a stretcher with an oxygen mask strapped to his face.
"It is awful. It was a very bad tackle. He had the oxygen mask on and was crying in the dressing room," said Van Gaal, whose promising young defender has broken his tibia and fibula.
Asked how long Shaw would be out, he added: "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."
Less than an hour after the injury Shaw tweeted: "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."
Shaw had been in top form this season after a difficult first year at Old Trafford following a reported 30 million pounds ($46.03 million) plus move from Southampton, which made him the world's most expensive teenager at the time.
He has started all of United's eight games so far this season and played for England in their Euro 2016 qualifiers against San Marino and Switzerland.
"A boy who came to United at 18 years old, it was very difficult, and the next season he plays fantastic and then that happens like this," Van Gaal was quoted as saying by the BBC.
"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."
PSV manager Phillip Cocu told a news conference: "We are all sorry about Shaw’s injury. Hector is a player who went for the ball and was not out to injure anyone but it didn’t look good.”
Moreno equalised for the home side in added time at the end of the first half after former PSV forward Memphis Depay had opened the scoring for United in the 41st minute.
Luciano Narsingh headed the winner for the Dutch champions just before the hour to finish off a dreadful night for United with their Spain goalkeeper David De Gea saying: "defeat hurts, but Luke Shaw's injury hurts even more".