United where trailing 1-0 when some of the lights went out at the Estádio Municipal, which led to play being stopped for around 15 minutes.
Asked if he thought the floodlight failure had helped United, Smalling – who was making his first appearance of the season having recovered from a broken metatarsal – said: "Yes, I think it did in a way. Especially after their goal. We knew we really had to pick ourselves up.
"We had a lot of the ball but we weren't really getting anywhere. So yes, it really helped us."
"He [Van Persie] is a person who only needs one chance," Smalling said. "That was crucial and he's proved that all season so far, and long may it continue.
"He [Rooney] is one of those who wants to put himself back in the firing line. I think he showed he's got the courage. You never have to doubt him really from the penalty spot."