COLLINGWOOD'S great ruck hope Brodie Grundy won't line up for the Pies in their NAB Cup opener against Essendon and the Western Bulldogs on Friday night due to a niggling back concern.

Grundy was restricted to walking laps with teammate Dale Thomas - who continues to be limited by his ankle problem - at training at Gosch's Paddock on Tuesday morning.

Assistant coach Matthew Lappin said he expected one or maybe two of the club's top draftees Ben Kennedy (pick No. 19) and Tim Broomhead (No.20) to play on Friday night.

But he was less confident about Grundy's status.

"Kennedy and Broomhead have done most of the pre-season, but Brodie Grundy's been a little bit limited with a back injury," Lappin told AFL.com.au.

Collingwood swooped on Grundy - regarded as the premier under-18 ruckman in Australia - with pick No. 18 in last year's NAB AFL Draft.

The Pies have carefully monitored the 18-year-old during his first senior pre-season, with senior coach Nathan Buckley saying Grundy needed to add size to his frame before he was available for selection.

"Brodie's on a modified program and he still needs to get his core strength up," Buckley said.

"He still needs to prove his fitness to be up and running in the next month or so.

"We've got individualised programs, and he needs a bit of work in the gym before we see him out on the footy field."

Thomas is still yet to resume full training and Buckley said he would need to progress significantly before coming in to selection calculations.

"When he gets back to full training, we'll probably need to see two weeks of solid work, so six main sessions of football, before his body is going to be in shape enough to throw him in to the hurly burly," Buckley said.

Buckley said Andrew Krakouer and Brent Macaffer will both play on Friday night, while the Pies will use the three-pronged ruck rotation of Darren Jolly, Jarrod Witts and Ben Hudson through the two games.

Krakouer and Macaffer, who will forever be linked through the torn ACLs they suffered at similar times during last pre-season, both impressed in last Friday's intraclub hit-out at Victoria Park.

Luke Ball, who also sustained a ruptured ACL last season, won't play on Friday night but is expected to be available for the later rounds of the pre-season competition.

Buckley said the Pies would look to develop both Witts and Grundy by giving them both senior opportunities at some point during the season, possibly at the expense of Hudson.

He confirmed recruits Clinton Young, Quinten Lynch and Jordan Russell would all don their new colours on Friday night.

But the coach sees the opening round of the NAB Cup as an opportunity to blood some of the club's youngsters, with a full squad to turn out against the Bulldogs and Bombers on Friday night.

Brodie Grundy is a ruckman in NAB AFL Fantasy. Register your team at our AFL Fantasy Hub.

Ben Guthrie is a reporter for AFL.com.au. Follow him on Twitter  @AFL_BenGuthrie