The Croatia international went under the knife in London this week in a bid to resolve a long-standing issue.
Initial estimates were that Jelavic would miss six weeks of Hull's battle against relegation from the Premier League, but Bruce believes the striker could be out for longer, revealing the 29-year-old striker has not been at 100 per cent for some time.
"If we're going to be truly honest about it, when he came back from the last operation we thought everything was OK, but it was pretty evident after a couple of weeks that it was troubling him," he said.
"That came to a head really with the basics of last week when he couldn't really train again and if you can't train day in and day out and you want to play in the Premier League, what you will do is get found out.
"For how brave he is and how tough he is, there's only really him who could have carried on the way he's been carrying on.
"I think last week was the end of it, when he said I need to go and have it looked at.
"The only way of doing that really was to go in and operate and that's what they've done.
"I hope, knowing Jela, he'll do his best to play a part but, if we're going to be realistic, we think it's going to be at least six weeks, which could force him out for the season.
"Knowing Jelavic the way I do, he'll give it every shot that he can be back."
Hull sit three points clear of the relegation zone and welcome leaders Chelsea to the KC Stadium on Sunday.
