Brighton midfielder Beram Kayal is expected to be out for at least eight weeks after breaking his leg in a pre-season friendly.
The Israel international, 29, sustained a small fracture in his fibula against Atletico Madrid on Sunday and had surgery on Wednesday.
Kayal played on before he was replaced at half-time during the 3-2 defeat.
"We don't expect any complications, but it's bad news for us and for him," said Brighton boss Chris Hughton.
The Seagulls start their maiden Premier League season against Manchester City at home at 17:30 BST on Saturday.
Kayal, who joined Brighton from Celtic in January 2015, was sidelined for two months last season with an ankle injury and his latest surgery is the eighth major operation of his career.
"The good news is it was a clean injury as such," added Hughton.
"The operation went very well - we expect him to be out for a period of something like eight to 10 weeks."
Former Newcastle and Norwich manager Hughton said he had not set any targets for his side after promotion to the Premier League at the end of last season.
"The outcome we'd like this season is to have a go and to make sure that in our first season in the Premier League we give it the best we can," he said.
"[We want to] have a real good go, and that we can have no complaints at the end of the season that we didn't do enough."