North Melbourne has been dealt another injury blow ahead of its high-stakes clash with Geelong this Friday night, with key defender Nathan Grima ruled out with a foot injury.

Grima missed last Saturday's clash with the Brisbane Lions and North coach Brad Scott confirmed on Tuesday that the defender would not play against the Cats.

Grima's unavailability comes a day after North announced that star midfielder Daniel Wells, who has missed the past three games with a foot injury, would be sidelined for up to another month.

The seventh-placed Roos have beaten Geelong in two of their past three clashes and Friday night's game is a great opportunity for them to consolidate themselves in the top eight.

North's prospects of beating the Cats should be boosted by the return of half-back Aaron Mullett, who was a late withdrawal against the Lions with a hamstring injury.

"Mullett should be fine, he's out there training, (but) Grima won't play," Scott said ahead of training at Arden Street.

"He's still too sore and we certainly don't want to take any risks with him.

"He's a really important player to us, so until he trains absolutely 100 per cent without any pain he won't play, and he's not training at all today so that rules him out this week."

Scott acknowledged Wells would be badly missed against the Cats, but said North aspired to be like the competition's best teams, which could cover the loss of one or two of their best players.

North did a good job of covering for skipper Andrew Swallow in the first seven rounds this year as he recovered from a ruptured left Achilles tendon, with star recruit Nick Dal Santo and Levi Greenwood stepping up in his absence.

Asked whether it was frustrating that he had not been able to assemble his best midfield yet this season, Scott said North would not be crying over "spilt milk".

"We've been working really hard on our midfield depth and we think we've got the numbers to cover [Wells]," Scott said.

"We've got to be better than just relying on one or two players, we've got to rely on 30-35 to get the job done this year."

North football chief Geoff Walsh estimated on Monday that Wells would return in three to four weeks, but Scott said North was "not quite sure how long" the two-time best and fairest winner would be sidelined.

But the North coach said a recent weight-bearing CT scan had finally revealed the extent of Wells' injury, revealing ligament damage in addition to the bone bruising the club originally diagnosed.

Scott praised the work North's medical staff had done with Wells and said his star midfielder was relieved that he now had a better indication of his likely return date.

"Our medical staff have been fantastic with Wellsy, because you can imagine the pressure they're under from me and the other coaches to get Wellsy back, particularly when we thought it was a short-term thing," Scott said.

"But there are different scans you can do – CT scans, MRIs, plain x-rays – but it wasn't until we did a weight-bearing CT scan that the extent of the injury became clear.

"So that's taken a lot of the stress away from Daniel and he knows what it is that's wrong and he's got a better idea of the timeframe to get back."