Mourinho has a long injury list to contend with and has already suggested that he will play the Under-23 side at the Emirates Stadium, as the game against Arsene Wenger's side falls between United's two-legged Europa League semi-final against Celta Vigo.
United will be without Luke Shaw, Marcos Rojo and Zlatan Ibrahimovic, who are all out for the season, while Ashley Young and Marcus Rashford picked up injuries in Vigo on Thursday night.
Mourinho says United have blown their last chance of finishing in the top four and the fact that Celta will rest players this weekend means his hands are tied.
But Guardiola says he doubts his old rival would ever throw the towel in.
"I am pretty sure that United is going to London to win the game," the Catalan told reporters.
"What I know a little bit about the times I faced my colleague Jose Mourinho, he is never going to drop even one game.
"It doesn't matter about the team he's going to play, his decision is he is going to travel to London to win the game."