John Kennedy hit out at Old Firm novice Nick Welsh as he laid responsibility for Celtic's heaviest derby defeat in 13 years at the feet of the debutant referee.

The Hoops crashed to a 4-1 loss at Ibrox after having Callum McGregor sent off just 26 minutes into the game.

The Scotland midfielder had only collected his first caution for a trip on Ryan Kent three minutes before.

Welsh waved play on as McGregor then crunched into Glen Kamara in the build up to Kemar Roofe's opener but returned to show the Hoops ace his second yellow as Gers celebrated.

It was a devastating double blow and while they did briefly level through Odsonne Edouard, Celtic were powerless to prevent Rangers racking up their biggest derby win since October 2007 as Alfredo Morelos, Roofe and Jermain Defoe piled on the misery.

After watching the fallen champions fail to register a single win over their bitter rivals a top-flight campaign for the first time in 21 years, interim boss Kennedy took aim at Welsh.

"Callum being sent off at the same time as Rangers score obviously swings the game in their favour," he said. "We got ourselves back into it but in key moments we weren't strong enough in defending our box.

"When we did have our chances, which are always going to be hard to come by with 10 men, we just couldn't get the ball in the net.

"It was a bit of what we have been (all season) again – but made all the harder because of the decision to send Callum off.

"I think it's a bad decision. Not the second one because he's on a yellow and makes a tackle on the ground.

"I asked the referee for clarity at half-time on why he booked him for the first one. His words to me exactly were 'it was a reckless challenge'. It wasn't reckless, I've seen it. There was no malice, no intent in terms of any power behind the tackle.

"It was a big game for a young official who doesn't have a lot of experience. He makes a very harsh call which costs us.

"Callum is gutted. He is the most disappointed man at Ibrox today.

"I feel for him because I don't think he deserved it. Okay, he makes a decision to go to ground (for the second one) which he'll learn from. But he shouldn't have been in that position in the first place, he shouldn't have had a yellow card earlier."

The match was Scott Brown's final Old Firm battle but, after 44 derby clashes, the curtain crashed down on top of the captain before he heads to Aberdeen in the summer.

But Kennedy said: "He won't be judged on that. It's not his fault. He has come here plenty of times and won. He's won many, many more trophies than a lot of people on the pitch today.

"Scott has had a terrific time at Celtic. This shouldn't be any sort of mark against him."

Having racked up a blemish-free campaign against their old rivals, Steven Gerrard's champions now just need to negotiate their final two games with Livingston and Aberdeen to complete an invincible league campaign.

The Ibrox boss said: "I'm really pleased with the performance.

"We started the game really well and picked a really aggressive and bold shape today as we wanted to go for Celtic's throat.

"We did that and took the lead but unfortunately we were sloppy in terms of the equaliser.

"But then again it was us on the front foot forcing the issue and then Alfredo scored a fantastic second.

"When that was happening there was also a big moment with the red card – but it was the right call.

"So we've worked extremely hard for this victory and we certainly deserve the three points.

"This club is about success. Seven days ago I was sitting here really stung and hurt (after being knocked out the Scottish Cup) as we'd missed a big opportunity.

"But in terms of the league I can't ask for any more. The consistency levels have been incredible."