Blackburn Rovers striker DJ Campbell faces up to two months out after damaging ankle ligaments in Tuesday's 1-1 friendly draw with Preston.
The 31-year-old, who played seven games on loan last season, after his contract expired at QPR.
He scored in the friendly before he was withdrawn as a precaution.
"He's out for a couple of months rather than weeks, he's in rehabilitation," Bowyer told BBC Radio Lancashire.
He added: "We'll be governed and guided by the medical team but it's not going to be the couple of weeks we had hoped for.
"It's disappointing for DJ after last season, he's looked fantastic and looked really sharp."
Campbell's loan stay was cut short by a broken toe last term, but agreed a two-year deal with Rovers.
Meanwhile, Bowyer also confirmed the club has withdrawn interest in Liverpool midfielder Jay Spearing.
"We put a bid in, we had it accepted and couldn't agree with the player so we've moved onto other targets," he added.