A tepid first period was enlivened by the winger's 42nd-minute introduction in place of the injured Phil Jones, and the entire tie ultimately hinged on the enforced substitution.
"His contribution was terrific," Sir Alex said. "Just after coming on he had two great strikes, one hit the post and the other one was saved. In the second half he got us the opening goal and I had the feeling he would win the match for us.
"I think he deserved it. He's a fantastic talent. You could see he was right on form when he came on. He's a good addition. If he keeps that form, it makes my job more difficult."
Nani's well-taken opener and sublime cross for Chicharito to put United two goals ahead seemingly ended Reading's resistance, only for Jobi McAnuff to ensure a nervy final few minutes with a poached finish.
"It ended up a really good tie. When they got their goal, they went for it," admitted Sir Alex. "They hurled the ball into the box, hoping to get that vital touch. But we withstood that. And I think when you analyse it, we played some great football.
"We could have scored a lot of goals tonight. We made some chances - we had shots blocked, the goalkeeper's made some saves, I'm really satisfied with the quality of the game. We really should have punished them better."
Sir Alex was speaking to MUTV and ESPN.