North Melbourne will almost certainly go into its round one clash against Adelaide without two of its best key defenders, but Michael Firrito is confident the Roos can counter the Crows' three-pronged forward line.

North will definitely be missing Nathan Grima when it takes on the Crows at Adelaide Oval on April 5, with the 29-year-old undergoing back surgery last Wednesday that is expected to sideline him for up to 14 weeks.

And Hansen is in extreme doubt after having surgery on both of his hips at the end of last season.

The 197cm defender is yet to return to full training and Firrito was already preparing for the likelihood Hansen would be unavailable for the Crows clash when he spoke to AFL.com.au on Friday.

It's never ideal to head into a game without two of your first-choice defenders, but the Roos' undermanned backline will be particularly tested by an Adelaide attack that is likely to centre around Taylor Walker (194cm), Josh Jenkins (199cm) and James Podsiadly (193cm).

However, Firrito was confident that the Roos had the depth to cover the absence of Grima and Hansen.

"It's absolutely shattering that Nathan Grima went in for back surgery this week and Lachy Hansen will miss the start of the year, but we're definitely going to get Lachy back and we'll get Nathan back later in the year too," Firrito said.

"It's very disappointing but Robbie Tarrant is absolutely flying, he's big and strong and moves really well.

"Joel Tippett has come back eight or nine kilos bigger and he moves really well too, and there's young Daniel Nielson as well.

"Once upon a time we would have been pretty worried if we were missing Hansen and Grima, but we've got a new-found depth and I think we'll be fine."

Tarrant, 196cm, has been training with North's defensive group this pre-season, having played most of his previous 39 games for the Roos in attack.

Firrito said Tarrant had been impressive in match practice sessions this summer and could play a similar role to Grima.

"Robbie moves really well, is a really good runner and is strong as well," Firrito said.

"I almost think they're like for like and Robbie Tarrant will come in and play that role across centre half-back but will also be able to go deeper as well.

Tarrant has endured a horrible run of injuries during his seven seasons at Arden Street, but Firrito says the 25-year-old has made an encouraging return from the tibia stress reaction that sidelined him for all bar one game in 2014.

"He's done the whole pre-season, he's fit, he's done the work," Firrito said.

"I guess there will no doubt in his mind that he's going to have a big year and North supporters hope he does too."