Anthony Caminiti has had his charge downgraded by the AFL Tribunal after striking Collingwood's Nathan Murphy during Round 5, given a three-week suspension after the tribunal ruled the incident as 'careless' conduct.

The young Saint was sent directly to the tribunal by the MRO, who graded Caminiti's behind-the-play strike as intentional conduct, severe impact and high contact.

Murphy, who left the ground soon after the hit, has been ruled out of Round 6's ANZAC Day clash after suffering a concussion.

St Kilda argued at the tribunal hearing that Caminiti's strike was careless rather than intentional and thus he shouldn't be facing such a lengthy suspension.

"As part of his way to defend me, he would instigate contact first ... as a forward, as part of my craft, I'm trying to get away from him and create space ... putting my forearm in his chest and therefore creating space between him and me," Caminiti said.

"I was anticipating the ball to get launched into the forward line. I momentarily looked at Murphy, attempted to put my forearm into his chest, anticipating a marking contest that didn't happen.

"At the time, I was completely unaware I'd hit him anywhere near the head or chin, I thought it was just a normal hit to the chest. Now looking at the vision, it's clear I must've accidentally clipped him in the chin.

"At other times during the game, the force (of that action) would've been the exact same ... I'd describe it as an attempted push to the chest with my forearm."

Caminiti also rejected the suggestion that his strike was in retaliation to Murphy hitting him first.

The AFL stated that the vision didn't support St Kilda's argument, instead insisting Caminiti should face a five-match ban if the tribunal deemed the strike intentional, four if the tribunal found the incident careless.

The vision of the action is inconsistent with the description given by Caminiti that he "lent his forearm into the chest," AFL legal representation Nick Pane said.

Caminiti's three-week suspension will see him miss St Kilda's upcoming matches against Carlton, Port Adelaide and North Melbourne.

The 19-year-old has played just five AFL games since being signed by the Saints via the Supplemental Selection Period earlier this year.