ST KILDA was on the verge of selecting Paddy McCartin in the team to face North Melbourne on Saturday night after his impressive recovery from a third bout of concussion this season.

Instead, McCartin will line up for Sandringham on Sunday afternoon at Trevor Barker Oval.

The 20-year-old won't wear a helmet after the club decided it would not offer much benefit, based on the available research.

Saints coach Alan Richardson said he was tempted to select McCartin in the must-win clash at Etihad Stadium.

"We'd expect Paddy to be back in the seniors reasonably soon. He's physically in really good nick," Richardson said at Moorabbin on Friday.

"He trained really positively on Thursday, so much so that it had us really questioning whether he could come back in.

"But on the back of the way the forward line functioned … the last two weeks, we've decided not to make a change."

Richardson had no doubt his side would improve when the No.1 NAB AFL Draft selection from 2014 returned. 

"He makes us a better footy team, so when he's right he'll come back in," Richardson said.

The match against North looms as the most pivotal of the season for St Kilda. Beating the eight-placed Kangaroos would bring the Saints within one win and percentage of the top eight.

The Roos will be motivated not only by wanting to keep the ninth-placed Saints at bay, but also by Brent Harvey's record-breaking 427th game.

St Kilda has not made finals since Ross Lyon was in charge, when the Saints lost an elimination final to the Sydney Swans in 2011.

For Richardson, this game will arguably be his most crucial match since his tenure began in late 2013.

"(It's) probably the most exciting opportunity we've had," he said.

"If we get a really positive result, the noise and the opportunity continues."

When these teams met in round seven, North Melbourne won by seven points. The Roos' tall forward trio of Jarrad Waite, Ben Brown and Drew Petrie was contained to five goals, although this time the Saints will have to limit the Roos without key defender Sam Fisher.

Fisher, 34, will join McCartin in the VFL, as the backman makes his comeback from a hamstring strain.

"The Kangaroos certainly pose a threat with three big fellas. They'd feel pretty positive about the fact that they've got big blokes, and I reckon would look to stretch us a bit," Richardson said.

"But we like the way the guys (in defence) have been working together.

"We certainly speak about it in meetings – we're going to be undersized (and we need) pressure on the ball.

"Our ability to put pressure on the opposition has been positive and we're going to need it again tomorrow."