TALENTED but underperforming forward John Butcher has been injected into Port Adelaide's line up to replace injured ruckman Matthew Lobbe.

Lobbe was finishing extra training on Thursday night when he injured his ankle, ruling him out of Saturday's game against Geelong.

Despite uninspiring SANFL form for the Port Adelaide Magpies, 21-year-old Butcher was called straight into the Power's starting 22 for his first AFL game of the season.

Butcher has managed only nine goals in his six SANFL games this year and has featured in the best players on just one occasion.

Drafted with pick No. 8 at the 2009 NAB AFL Draft, Butcher has failed to live up to his promise at Alberton after bursting onto the AFL stage with 11 goals in his first four games in late 2011.

Injury has held him back, the latest a broken hand suffered on the eve of the season.

Assistant coach Shaun Rehn said picking the injury-prone forward was like walking "out on a twig", but said his SANFL form was also poor before his eye-opening AFL debut.

"Form in regards to ball in hand hasn't been great for 'Butch', the effort's been there ... yes, there is some risk, but with risk is also great reward. Hopefully for Butch we'll get the reward," Rehn said.

"Kenny (Hinkley) termed it, you walk out on a twig a bit with Butch.

"He's been training and he's been playing without any restriction now for many weeks. I think we should also remember Butch's form before he came in at the end of 2011 was also not great at SANFL but came in and had a real impact."

Lobbe's injury leaves Jarrad Redden as the club's only recognised ruckman again, after he played the same role against Richmond in round seven.

Rehn said Justin Westhoff would again be forced to help out in the ruck.

With Geelong ranked last in the AFL for clearances and 14th for hit-outs, Rehn said Saturday's game was a good opportunity to Redden to assert himself against the Cats' ruck pair of Trent West and Mark Blicavs.

"Statistics are not always what they're built up to be, [but] at the same time Geelong and us are right down the bottom in regards to hit-outs as well [as clearances]," Rehn said.

"There's definitely a small correlation between getting your hand on the footy in the ruck and your ability to clear the ball.

"We'd like to get the advantage [in clearances]…it's a good week for our young ruckman in 'Reddo' to see what he can do against Geelong's ruckmen."

Harry Thring is a reporter for AFL Media. Follow him on Twitter: @AFL_Harry