Both senior stars are out of the big game.
"We've had big games before [but it is] certainly a test for us," said Di Matteo. "They are two influential players for us and we are going to miss them on Sunday, so I think all the others will step up and we have to show we are a team with character, with personality and we are showing the personality to play in these big games.
"Games against Man United were always very competitive and quite balanced, enjoyable games you looked forward to. It was playing against the best players in the league at the time, and you always looked forward to those games."