Lazaar looked to have pulled his hamstring in Tuesday night’s 2-0 win over Brentford at Hillsborough in his second start for the Owls.
Speaking to the Star ahead on Monday’s derby match, Bruce said “We actually won’t know the severity of it until this afternoon when he will have a scan.
“You have to wait 48 hours until you can re-assess it really so we’ll have to wait until this afternoon to know but you have to stay the early stages don’t look good for the boy to be perfectly honest.
“Fingers crossed that it’s not as bad as we fear. We have had ten injuries the other night and now with Lazaar all of a sudden it is ridiculous numbers.
“So it’s something which I have to address but in terms of the style of play that we want to try and implement, I think the players have enjoyed what they are doing. Of course when you get a couple of results, everybody enjoys it so long may it continue.
“The lad was beaming, because he played and he played well on his debut but I’m kicking myself a little bit.
“Weeks, but at least six I would have thought.”
Lazaar stepped in at left-back for the injured Morgan Fox, who was sidelined before Saturday’s match against Swansea with a back spasm.
But Bruce confirmed that Fox and fellow defender Jordan Thorniley, who had been absent as he recovered from concussion, were on their way back to match fitness.
He said: “It is something that I have to be wary of because different training methods is a bit different to what they are used to so I’m going to have to box a little bit clever if I can.
“It’s something that I have to address the amount of injuries that the club has picked up and where we have been with long term injuries.
“I don’t know a club like it. You can have one but we have had four or five which is not very good.
“I think Fox is looking to join in training, we’ll see how he is today. He still a little bit ginger with it but we hope he is going to be ok.
“Jordan, we are monitoring him. The ones that you can probably reel off are still there.”