"He is perfect and ready to go," Clarke said when asked about the fitness of the Napoli man, Scotland's star player.
McTominay, 29, is coming off a season in which he scored 14 goals in all competitions for the Serie A runners-up.
He has become an icon in Scotland since his stunning overhead kick goal in the crucial 4-2 win over Denmark last November which sealed his team's qualification for a first World Cup since 1998.
But Clarke does not want to place too much pressure on the versatile former Manchester United star heading into the Group C opener in Boston.
"I think I have got 26 superstars here. To try and put so much onto one person is not fair. We have built everything we have over the last seven years on the squad, the team, everybody being together and playing their part at certain times," said Clarke.
McTominay has been used in a variety of roles in a Scotland shirt but should play as an attacking midfielder at the World Cup.
"Some daft coach put him as a centre-half five years ago, but obviously he is not a centre-half," added Clarke.
'Don't get humped'
"Since he moved further forward he has done great for us. We are delighted with Scott's abilities and what he can bring to the team, but we are going to need another 15 that can do the same if we want to have a positive tournament."
Scotland need to get off to a good start as they attempt to get out of the group stage at a World Cup for the first time, with Morocco and Brazil, ranked seventh and sixth in the world respectively, to come next.
Reaching the tournament comes off the back of Scotland making it to the last two European Championships, both of which they exited in the group phase without winning a game.
"The last two tournaments have not gone the way we wanted them to go, but we have another chance, and that is credit to the players that they keep qualifying for these major tournaments. It is great to be here, but we also want to do something special," said Clarke.
The veteran 62-year-old coach has been in the job since 2019 and last month signed a contract extension until 2030.
He was asked what lessons his team could learn from the start of their campaigns at the last two Euros, including in 2024 when they were hammered 5-1 by hosts Germany in the tournament's opening game.
"The Germany one is easy, don't get humped," he joked.
"The way we started that tournament, we let ourselves down and then you have that bad feeling hanging over you the whole tournament."
FIFA World Cup 2026
The 2026 World Cup will be held from June 11th to July 19th in the United States, Canada and Mexico. The tournament will feature 48 national teams and will be played in 16 modern stadiums.
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