Dutch centre-back Loovens, who made 38 appearances for the Owls during the 2015/16 campaign, recently had ankle surgery and is set to be out of action until September.
His absence leaves Wednesday short of defensive cover. Utility ace Sam Hutchinson has been utilised in that position throughout their pre-season programme.
Boss Carlos Carvalhal has made it clear he wants to bring in one new centre-half before the transfer window closes on August 31.
Westwood, speaking exclusively to The Star at the Owls' training base in Portugal, said: "It's a big blow losing Glenn.
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"He's the captain of the team and been a very good player for us. It's a big shame and I'm a really disappointed.
"We can't cry over spilt milk. We have just got to get on with it.
"Sam's a midfielder or a right-back but he's good enough to slot in anywhere which is a massive bonus. He showed he can play there last year.
"Sam brings real presence to the team. He gets his head on things and is really comfortable on the ball which is great."
It has been a hectic summer for Westwood. Having played an integral part in the Owls' charge to the Championship Play-Off final, the former Sunderland player spent five weeks away on international duty with the Republic of Ireland.
Despite being placed in a tough-looking group which included Belgium and Italy, Martin O'Neill's reached the last 16 of the tournament. They bowed out of the tournament with their heads held high, giving host nation France a real scare.
Westwood said: "It was great. I think we did the country proud. It lifted everybody's spirits.
"I'm sure there must have been 100,000 Irish people in France!
"We got praised for the way we acted and the way the fans behaved themselves. There was no trouble or anything like that with the Irish fans but that's Irish people to a tee.
"We did everybody proud and it was nice to be part of the squad. I love getting called up and being part of the Irish team and set-up and that's never going to change. It was great to be involved."
Westwood is pencilled in to feature in Wednesday's pre-season friendly against CD Nacional tonight, kick-off 8pm. It will be his first outing since the Owls' heart-breaking Wembley defeat last May.
Possible Owls line-up: Westwood; Hunt, Lees, Hutchinson, Palmer; Wallace, Lee, Bannan, Matias; Joao, Forestieri