The Australian prop was red-carded for punching North Queensland Cowboys’ James Tamou late on last night as the Super League champions were crushed 38-4 at Headingley.
It was the first time anyone had been dismissed in the fixture since Manly’s Ronnie Gibbs - the former Castleford second-row - against Wigan in 1987.
Garbutt reacted after Leeds team-mate Keith Galloway was first struck by Tamou.
McDermott said: “There’s some provocation but not enough.
“It was silly from Mitch. He was very passionate in the second half - a lot of our players got a bit frustrated - but it was silly from him.
“It didn’t justify that. I’m sure Keith can handle being slapped.”
Leeds are already depleted, missing seven first-team players to injury last night, and Garbutt could now face a potential suspension ahead of Saturday’s game at Catalans Dragons.
McDermott had no complaints with the defeat last night, the game long lost before that red card.
Rhinos had shown great spirit to go in at half-time level at 4-4 but the Cowboys scored within seconds of the restart and never looked back.
“That was a big moment for us in the game - to keep them down just not that single try there,” said the Leeds coach.
“It probably took bit of juice out of us and we needed to get a foothold in the early stage of the second half but didn’t.
“Ash Handley dropped a kick - he’d done great as they peppered him all night and he dealt with most of them - and we were poor in the first 10 or 15 minutes of that second period.
“We allowed them to be as good as they could be which was very good.
“I thought the physicality took its toll during the game.
“When their big fellas carried up the middle and backed themselves we didn’t match them physically.
“In games like these in the past, we’ve probably been unpicked by some really smart plays but this time, although we did a good job, physically we started dropping off with all that effort.
“It’s a difficult one when they get a roll on.”
McDermott wanted Leeds to offer more in attack but they were blunted by the NRL premiers who won their first World Club Challenge and completed a second successive 3-0 whitewash for the NRL sides this weekend.
“We didn’t throw a lot at them, “ he said. “We backed ourselves and we were full of vim but we could have challenged them more with the ball.
“As for positives, I might be able to say something Monday.
“But we’ve just lost and conceded 38 points.
“That’s really disappointing and the scoreline did reflect the game.
“You might say we were brave, and a kick ricocheted back for one of their tries, then there was that one with Ash, but they were the better side and we know that.
“You look at some of our players, though, and Liam Sutcliffe and Jordan Lilley, have a combined wage of about £32,000.
“Then you compare that to some of their top-line players.
“We gave it a really good go. People will say this is another whitewash for Super League and question whether it should carry on.
“But there is some real good merits in this and that’s how you learn, if we get the chance again we’ll give it an even better crack.”