Cueto, who only returned to international rugby during this year's Six Nations following a 16-month absence, says the news that he had not made McGeechan's final cut has hit him hard.
"I am massively disappointed not to get the call-up," said the 29-year-old.
"It's all I've thought about since the squad was announced last Tuesday.
"I'm probably more gutted than I've let on to people. But from a personal point of view I don't think I could have done a lot more."
Despite the obvious setback, Cueto knows more than most that his initial disappointment could yet turn to relief, albeit as a result of someone else's misfortune.
The Sale regular missed out on selection for the Lions four years ago, only to be called into the party after injury ruled out Iain Balshaw prior to departure for New Zealand.
Without wishing ill on any of his fellow professionals, Cueto is fully aware that he could still yet make the plane to South Africa.
"There are a lot of games between now and the end of the season for some players.
"I'd never want injuries for other players, but that is what happened four years ago and I took my chance.
"Aside from the Lions I hope to be involved in the upcoming England games, and I have to crack on with it.
"We play Argentina at Old Trafford and as a Manchester United fan, you can't put a price on what that would mean to me."