Sam Allardyce said: "Henrik has felt soreness throughout the summer and we feel an exploratory operation is the right thing to do.
"We will have a better understanding of the problem later this week."
Meanwhile, Bolton boss Sam Allardyce says an agreement is close with out of contract Lecce defender Lorenzo Stovini.
"Talks are ongoing and I'm hopeful a deal can be concluded later this week," Allardyce told the club's website,
"He is very experienced and should add more depth to the squad."