The Italian said that his side showed "fantastic character" to see off the Red Devils at Stamford Bridge and book their place in the quarter-finals.
"I thought it was a very exciting game. We played well and had a lot of chances," BBC Sport quotes Di Matteo as saying.
"We could have finished off the game sooner than we did. The boys showed they have fantastic character.
"Jon Mikel Obi and Juan Mata, who was magnificent, were in the right frame of mind to play," he added.
Chelsea will take on Leeds United at Elland Road in the last eight of the competition.