The 71-year-old hails the Portuguese winger's impact coming off the bench for Phil Jones as he helped the Red Devils to a 2-1 win over Reading in the FA Cup fifth round
The Portugal international came off the bench shortly before half-time for the injured Phil Jones and went on inspire the hosts to victory.
The 26-year-old struck the post before finding the net after the break to put United ahead. He then helped double the hosts' lead with an assist for Chicharito's goal.
"His contribution was terrific," Sir Alex told the club's official website. "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."
Jobi McAnuff pulled a late goal back for the Royals to set up a nervy finish but Sir Alex was pleased in the manner his side held on.
"It ended up a really good tie. When they got their goal, they went for it," the Scot added.
"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. 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."