SCOTT Selwood's ambition to become the AFL's best defensive midfielder will face a litmus test on Saturday night, with the young West Coast stopper set for a crucial match-up with one of Carlton's star on-ballers.

Selwood, who has this year emerged as one of the game's leading shut-down specialists, said he was eager to take on either Chris Judd, who was announced on Monday night as the 2011 AFL Players' Association MVP, or Marc Murphy in the knockout semi-final at Patersons Stadium.

Selwood worked in tandem with Daniel Kerr to hold Judd to 17 possessions when the sides last met in round 14 at Etihad Stadium, playing a key role in West Coast's 36-point win.

He said the Eagles would consider whether to target Judd or Murphy on Saturday night, with the latter racking up 37 possessions in a best-on-ground performance against Essendon last Sunday.

"I think he (coach John Worsfold) knows I'm pretty eager to get one of the two," Selwood said.

"It doesn't matter who it is - I'll give it a good crack and see how we go. That's my role for the side.

"They're a good combination, so we'll assess who we find the most dangerous and who we'll probably have to tag this week. They've both been in really good touch."

Selwood has taken a big step in his fourth AFL season, leading the competition for tackles and pressure acts and averaging 18 possessions a game as a defensive midfielder.

While midfield coach Scott Burns says Selwood has "too many tricks" to remain purely a run-with player, the 21-year-old is setting himself to be the game's best midfield stopper for now.  

"I feel as though my defensive game is one of my biggest strengths and I did want to become probably the best defensive player through the midfield in the competition," he said. 

"I feel as though I'm very close to that, but that just means I've got a lot of things to work on on my offensive side as well.

"I set certain goals out and I want to achieve them. If I achieve them, then I'm happy. If I don't achieve them then it just gives me more drive to try and go for next season or next game or next contest."

Selwood said Judd was the strongest player he had faced, with the Carlton skipper gaining an advantage on him through his body-use at stoppages.

He said his approach if given the task on Judd would be to break the game into small intervals.

"You have to try and break it up into five-minute blocks," Selwood said.

"He might win one small area and then you go off on the next little bit, the next five minutes, [and say], 'I'm going to shut down on him as much as I can'.

"If you win that one then you say, 'All right, I'm going to try and win some footy now'.

"There's different little techniques that happen throughout the game that you try with different players, but with someone like 'Juddy' you always feel as though he's dangerous, so you never want to leave his side too much."

Nathan Schmook covers West Coast news for afl.com.au. Follow him on Twitter: @AFL_NSchmook