Manchester United's new £18.6m signing Michael Carrick is set to miss the start of the Premiership season after injuring his left foot against Ajax.
The midfielder lasted just 14 minutes on his second appearance before getting injured in a tackle from behind.
"It looks like he could be out for three weeks, there's a lot of swelling," said boss Sir Alex Ferguson.
Ryan Giggs' free-kick gave United a 1-0 win and meant they were the winners of the four-team LG Amsterdam tournament.
Ferguson added: "Michael has ligament damage to his ankle and it is difficult to say how long he will be out because we don't know his record with injuries.
"The last two years have been horrendous for us in terms of injuries"
"But it looks like he could be out for three weeks and it is a real blow for the young lad. He wanted to do really well for us and this has knocked him back a bit.
"But we have to handle it. Injuries are part of the game and we have plenty of good players in the squad who can take his place.
"I'm not going to complain about the challenge, though, because the referee showed tolerance tonight and that's what you want.
"You don't want to see players sent off in pre-season friendlies."
Carrick's debut against Porto on Friday was marred by sendings off for Wayne Rooney and Paul Scholes, who could now collect bans from the FA.
Manchester United's first league game is against Fulham on 20 August and Ferguson already has several injury worries with Gary Neville, Gabriel Heinze and Nemanja Vidic all currently sidelined.
Rio Ferdinand and John O'Shea both picked up injuries against Porto although both should be fit for the start of the season.
"Your hope for the start of every season is a clean bill of health," said Ferguson.
"The last two years have been horrendous for us in terms of injuries. I was hoping we would get a free run this year but it hasn't happened.
"But we have a big squad and we have to get on with it. We have great experience and hopefully that will give us a good chance."