Porto defender Christian Sapunaru is a major doubt the first leg of the UEFA Champions League tie with Manchester United on Tuesday.
The 25-year-old injured his ankle in the 3-1 win over Vitoria Guimaraes at the weekend.
The Romanian is set to have a scan on Monday morning to determine how serious the problem is.
Sapunaru has featured three times in the Champions League this season including the 4-0 defeat to Arsenal in September.
Better news for Porto is that midfielder Freddy Guarin has returned to full fitness and has been included in squad for the Old Trafford clash.
The Colombia international has fully recovered from an ankle sprain and is set to feature along with Lisandro Lopez who was suspended for Saturday's league victory.
Porto captain Pedro Emanuel and Nelson Benitez are both still sidelined with injury as is Jorge Fucile, while teenager Rabiola has been left out of the squad.