The Star understands Loovens went under the knife in London yesterday for a long-standing ankle injury.
Now the Dutch defender is expected to be out of action for six to eight weeks, according to boss Carlos Carvalhal.
Speaking after Wednesday drew 1-1 at Alfreton Town in their opening pre-season friendly last night, Carvalhal told The Star: “It is, of course, not good news for us because Glenn is the captain of the team and an important player.
“It is a small problem that he felt last season.
“It is a surgery so he will be out a minimum of six to eight weeks.”
“It is not the end of the world because he will be back with us soon. He has a very good personality and is a fighter.”
Strengthening their defence is at the top of Carvalhal’s priority list.
“We don’t have many centre-backs or left-backs,” said the Portuguese chief, who has confirmed he has received no offers for their top stars this summer. “We have holes in those positions.”
Striker Fernando Forestieri was a notable absentee from their draw at Alfreton where Barry Bannan opened the scoring.
“Fernando had a small pain and we didn’t take any risks,” said Carvalhal. “If he had played, maybe he stay one or two weeks out.”
Keiren Westwood will resume first-team training today but is unlikely to be involved in Saturday’s friendly at Chesterfield.
Meanwhile, winger Ross Wallace has penned a contract extension, tying him to the Hillsborough outfit until 2018. The winger racked up 49 appearances for the Owls last season, hitting six goals in their march to the play-off final.