Crystal Palace will have to do without Gallagher at Wembley because he is on loan from Chelsea and cannot play against them.
The 22-year-old has been one of the Eagles' best players this season, scoring eight goals and providing five assists in 32 appearances across all competitions.
Gallagher's form has seen him rewarded with call-ups to recent England squads.
Palace tried to get Chelsea to allow Gallagher play under a special dispensation but the Blues denied the request.
Tuchel acknowledges Gallagher's frustration of missing out but cannot stand to see him hurt Chelsea's chase for domestic honour.
"I had the chance to speak to Conor and I could also see his frustration," Tuchel said ahead of the match. "We met some weeks ago, after the international break, by coincidence in a restaurant.
"We had a chat, and the subject came up. I apologised, because I know how competitive he is, and I like him a lot. I know his character very well, and it was a pleasure to have him here in pre-season.
"This is the way it is. We play to win the game. The rules were clear when we made the loan.
"I can totally understand Patrick Vieira and the disappointment of Conor, but these were the rules when we made the loan, and we don't want to suffer from our rules."
