The versatile Northern Ireland international signed a new three-year deal with Rovers a week ago but suffered his injury in the friendly at York City last Tuesday.
McCullough twisted awkwardly while attempting to track a runner and a scan has revealed he has cruciate knee ligament damage.
"He's going to be out for the season," Ferguson said,
"I really feel for the boy. He's just signed his new contract and then he's done his cruciate.
"He will be out for the season certainly.
"These things happen. The boy has got to stay strong.
"We'll help him with his rehab but now he's got to take the advice of the surgeons.
"You can't rush these things. It's probably the worst injury you can have."
Ferguson had earmarked McCullough as a crucial player for Rovers this season with the 22-year-old set to occupy a defensive midfielder role.
He said: "He's a big player for us because of the way he plays.
"In this league in that role, he's probably the best player if I'm honest."
McCullough's injury is the worst so far in a catalogue of pre-season injuries for Rovers, who could be without eight senior players for Saturday's opening game at Accrington Stanley.
But Ferguson is refusing to be downhearted by the latest in a series of blows.
He said: "It's unfortunate but it's an opportunity for someone else now.
"I have other players who can play there.
"It's a blow for us but we'll have to carry on with it.
"I'm prepared for most things. It's been a bit testing this pre-season but I don't want to keep going on about it.
"I've got to make sure everyone in the squad is strong and ready to go."