The 30-year-old defender, who won the Serbian title with Red Star Belgrade last season, signed a short-term deal with the Owls after impressing on trial in the summer.
He picked up a knee injury in pre-season against Newcastle United, but was deemed fit enough to be named as a substitute for the opening-day trip to Brighton. But scans have revealed he has ruptured his anterior cruciate ligament and he has been ruled out the for season.
“It’s a bombshell for Dejan,” said Owls’ head coach Stuart Gray. “I’m so gutted for him. He’s just been very unlucky.
“He’s distraught. It was just an innocuous challenge in the Newcastle game. He’s tried training with it. He was on the bench at Brighton and suddenly his knee has blown up. Unfortunately it’s not good news for him or us.
“We sent him off for a scan and it didn’t come back a good report. He’s hurt his ACL so he’s going to have major surgery. The specialist says he’s torn the ligament in his knee which needs to be repaired.
“I can’t see him, unfortunately, playing for us again this season. It’s the curse of the centre-halves. It seems to have carried on from last season.”
Kelhar’s absence means the Owls are down to three centre-halves, in first-choice pairing Glenn Loovens and Tom Lees, plus fit-again Kamil Zayatte, who missed the second half of last season through an ankle injury.
But Gray hopes to avoid recruiting another defender before the transfer window closes, with his priority being a striker and winger.
“At least I have got a little bit of depth there,” said Gray. “Kamil came on for Glenn the other day and showed his qualities.
“Last Saturday, I had Dejan and Kamil on the bench. I’ve got two outstanding centre-backs who are maybe performing because they can see what’s waiting to come in and take their place.
“Luckily I played Sam (midfielder Hutchinson) in pre-season there a couple of times. I know Sam can fill in.
“What I don’t want to do is take away from the midfield what Sam brings.
“If I have to, I will do. Lewis (left-back Buxton) and Semi (midfielder Jose Semedo) can fill in there but it’s not ideal because I like round pegs in round holes.”