Sunderland expect defender John O'Shea to miss Saturday's Barclays Premier League opener against Liverpool.
The versatile Republic of Ireland defender was signed from Manchester United this summer but has made only one appearance in his new club's pre-season campaign, injuring a hamstring on his debut against German side Arminia Bielefeld.
Manager Steve Bruce told the Black Cats' official website, www.safc.com: "We don't think (he will feature) and it would be unwise of me to rush him back."
He continued: "He's close, but he's got a hamstring strain so we have to make sure he's right.
"When he is right and plays we'll be all the better for it because John O'Shea is a big acquisition for this football club."
And Bruce is not ruling out further additions despite wrapping up a deal to make Derry City winger James McClean his 10th signing of a busy summer.
O'Shea and United team-mate Wes Brown joined Birmingham midfielders Craig Gardner and Sebastian Larsson, wingers McClean and David Vaughan, strikers Connor Wickham and Ji Dong-won and Coventry keeper Kieren Westwood in arriving at the Stadium of Light.
A permanent deal was also agreed for former loanee Ahmed Elmohamady.
Bruce said: "The squad size is good at the moment - I'm very pleased with what we've done.
"But as James McClean has shown, a manager's work is never done. If there's someone out there who we feel would be a good addition to the squad then we are in a position to act."