Mainoo failed to add to his 14 international caps this summer as he missed the clash against France on Saturday night due to injury.
The Manchester United midfielder had not played a single minute under Tuchel before the clash as the likes of Declan Rice, Morgan Rogers and Elliot Anderson found there place above him in the pecking order.
Mainoo, one of the nominees for the Premier League’s Young Player of the Year award, was confirmed to have picked up a knock after the game by Tuchel who looked worried about the 21-year-old.
"Kobbie was injured in the last training session yesterday morning," the German tactician told reporters.
"He had a sharp pain in the back within the training and he was not ready to play."
Mainoo will now have to wait four years before having another chance to make his World Cup debut in what is a devastating blow.
United boss Michael Carrick may also be worried as the Premier League kicks off in one month’s time as the Red Devils open the season with an away clash against newly promoted Hull City at the MKM Stadium.
Carrick’s side, who suffered a 1-0 defeat to Wrexham in their pre-season opener, may be worried as Mainoo seeks to return as soon as possible.