Rashford helped United overturn a 2-0 deficit to win 3-2 on Monday night.
Mourinho said: "I think Marcus [Rashford] was good and immediately he created dangerous situations and immediately he opened more space in between defenders. Our attacking players could make runs and penetrate in these gaps, so by the positional point of view, he was very good for us and then I think what I call the crazy right-back was also very important.
"We had to take the risks and [Jesse] Lingard as a right-back is this kind of crazy right-back that can make a defensive mistake by losing his position, but he can also be an extra man attacking other areas.
"I think it was difficult for them [Crystal Palace], Roy [Hodgson] saw it, he then brought [Alexander] Sorloth to play wide and try to cover him, but he was really aggressive and [Juan] Mata came to the inside so I think we created them a lot of problems in the second half."