Sunderland boss Martin O'Neill fears Republic of Ireland defender John O'Shea could be a doubt for Euro 2012.
The player aggravated a calf injury late in the first half of Sunderland's defeat by Manchester United.
"John didn't feel comfortable. He was starting to feel his injury, so he just didn't feel confident at all about moving on it," said O'Neill.
"I haven't had a chance to check with the physios since. But it might be a concern [for the Euros]."
The experienced O'Shea, 31, is a key man in the Ireland squad and his loss would be a huge blow to boss Giovanni Trapattoni as he leads the Republic into their first major tournament since 2002.
O'Shea sustained a calf strain against QPR in March but returned to play in Sunderland's last four games of the season, before pulling up in Sunday's match.
Asked if O'Shea's injury could jeopardise his chances of going to Ukraine and Poland this summer, O'Neill added: "It might do.
"As he came off, that was my thought, that there might be a concern about it. But maybe I am jumping the gun."
O'Shea and the rest of Trapattoni's Premier League-based players are due to meet up in Malahide next Sunday to begin their preparations for the tournament.
The Republic's opening game is against Croatia in Poznan on 10 June.