Warren Gatland said concerns over Johnny Sexton's "durability" lay behind one of his "toughest decisions" in omitting the veteran fly-half from the upcoming British and Irish Lions tour of South Africa.
Ireland captain Sexton was the most high-profile absentee from the 37-man squad unveiled by Lions boss Gatland on Thursday, with the New Zealander opting for Wales' Dan Biggar, England's Owen Farrell and Scotland's Finn Russell as his three No 10s for the July-August campaign.
Sexton, a veteran of 105 Tests, has featured in the Lions' last six internationals spanning the 2013 series win in Australia and drawn encounter in New Zealand four years ago.
He also impressed for Ireland during this season's Six Nations.
But Sexton, whose career has featured several head injuries, has since suffered concussions.
While Gatland said he had a "huge amount of respect for Sexton", 36 in July, worries about his ability to get through a tour culminating with three Tests in as many weeks against the physical, world champion Springboks had counted against him.
"It was one of the toughest decisions we had to make," Gatland told reporters.
"When we looked that we are going to South Africa, the thing that we kept coming back to is Johnny's durability and being able to put a string of big matches together consecutively," the New Zealander added.
"He's been rested on a number of occasions and unfortunately that's what swayed it for us –- not having the confidence that he could get through a tough, physical tour."
- 'Exciting Simmonds' -
By contrast, Exeter No 8 Sam Simmonds is in the squad even though the reigning European player of the year won the last of his seven England caps in 2018.
"For the Springboks you've got to have players in your forward pack who don't just put the ball under their arm and run hard and straight," Gatland said.
"You need players with footwork and Simmonds has that and pace. He also scores tries. I'm really excited about him getting the chance."
South Africa have not played a Test since winning the 2019 World Cup final in Japan because of the fall-out from the Covid-19 pandemic.
But Gatland, ahead of his third tour as Lions head coach, was in no doubt of their quality.
"I think South Africa have gone back to their DNA," he said.
"It's about being physical and winning that physical battle...It's going to be brutal at times."
Prior to a warm-up Test against Japan at Murrayfield on June 26, the Lions will hold a training camp in Jersey.
A self-governing territory dependent on Britain for its defence, Jersey has been at the centre of a post Brexit row between the UK and France over fishing rights.
London recalled two navy vessels on Thursday while protesting French fishermen returned home without blockading the Channel Island's main port.
"I'm only concentrating on the rugby side, not the gunboats and the other stuff that's going on," said Gatland.
"We've just got to make sure that if we do go fishing, we don't go out too far," he added jokingly.