Sir Alex told MUTV on Thursday: "Antonio was back to his best [on Tuesday]. Changing the number of his jersey again - it's amazing how it can affect a player like that.
"Maybe it was a great thing for him to do that. He used his initiative to go back to 25 and was fantastic."
Valencia also confirmed that his celebration on Tuesday night was a tribute to former Ecuador team-mate Christian 'Chucho' Benitez, who sadly lost his life at the tender age of 27 in the summer.
"It was terrible that he passed away a couple of months ago," stated the winger. "He wasn't just a friend but like a brother to me and I dedicated the tattoo to him. That first goal celebration and every other goal that I celebrate hopefully this season, I'll be dedicating to him also. It was just a huge shock to everyone."