Geelong will be without star defender Tom Stewart for their next four AFL games after failing in their bid to reduce his rough conduct suspension.

As expected, Stewart pleaded guilty on Tuesday night after he was referred directly to the tribunal for the ugly bump that concussed Richmond onballer Dion Prestia.

Stewart will miss Geelong’s games against last-placed North Melbourne, defending champions Melbourne, fifth-placed Carlton and 12th-placed Port Adelaide.

Geelong are second after beating the Tigers, with Stewart knocking out Prestia late in the first quarter on Saturday night at the MCG.

Prestia was forced out of the game and the ugly incident reignited debate of whether a sin bin or send-off rule is needed in the AFL.

The Cats did not contest that the charge was careless conduct, severe impact and high contact.

They argued Stewart had .39 of a second before he took out Prestia and pointed to his spotless playing record, as well as his genuine remorse.

But tribunal chairman Jeff Gleeson and fellow jury members Jason Johnson and Paul Williams ruled the degree of carelessness was high.

“He ran past the ball and made a conscious, albeit split-second, decision to bump player Prestia at speed. The degree of carelessness was high, he breached his duty of care by a significant margin,” Gleeson said in the tribunal verdict.

“While he did not intend to commit a reportable offence, he was careless as to whether he did so.”

Gleeson added the bump was late and happened well after Prestia would have expected contact.

“He was wide open, exposed and vulnerable,” Gleeson said.

“Player Stewart had sufficient time to see that player Prestia had not taken possession and was no longer actively involved in the contest.”

Stewart’s advocate Ben Ihle raised Sam Reid’s two-game ban and Patrick Dangerfield’s three-match suspension last season on rough conduct charges as they argued for a three-game penalty.

But those incidents did not sway the tribunal.

“Neither example is sufficiently comparable to provide close and clear guidance,” Gleeson said.

AFL advocate Nicholas Pane said the bump was delivered at speed and Prestia was vulnerable when Stewart made impact, adding the concussion was “a serious injury in itself.”

Stewart will serve the first suspension of his AFL career and had conceded before the hearing he was “at the mercy of the tribunal” and indicated he would accept his punishment.

Richmond’s medical report on Prestia, which was tabled in the tribunal hearing, said a CT scan had cleared Prestia of any facial fractures from the bump.

But he will miss a game at least because of his concussion.