The Manchester United midfielder had yet to play a minute at the tournament and had been expected to feature in Miami as Tuchel rotated heavily for the bronze-medal match, handing minutes to several fringe squad members as England went on to win 6-4. Instead, Mainoo was left out of the squad entirely, with FIFA listing him as injured on the official teamsheet.
Jordan Henderson, who broke his wrist in a freak accident following the last-16 win over Mexico, was still named among the substitutes despite his own injury, while the FA also issued a statement separately confirming Mainoo's problem.
Tuchel gave the fullest explanation yet of the issue on Saturday, revealing the 21-year-old had been struck down during England's final training session before the game.
"Kobbie was injured in the last training session (on Friday) morning," Tuchel said. "He had a sharp pain in the back within the training and he was not ready to play."
The setback means Mainoo ends the World Cup without having featured for England at the tournament, despite being part of the squad throughout. His injury adds to a list of fitness concerns for England as the squad returns from the United States, with the full extent of the issue and any recovery timeline yet to be confirmed by Tuchel or Manchester United, who will assess him upon his return to pre-season training.
Mainoo had been expected to be one of the more high-profile beneficiaries of Tuchel's rotation policy for the third-place play-off, having struggled for minutes throughout the tournament behind England's first-choice midfield pairing.
His absence from the matchday squad entirely, rather than simply being an unused substitute, underlined the seriousness with which the England medical staff treated the injury in the hours before kick-off.
United will now be keen to establish the severity of the issue before their own pre-season schedule intensifies, with Mainoo expected to play a central role for Michael Carrick's side once he has recovered.