United had just fallen a goal down to Alan's 49th minute penalty when play was stopped for 15 minutes at Estadio AXA with parts of the pitch in darkness.
But the unscheduled interval proved to be a spark for Sir Alex Ferguson's side, and they turned the game on it's head in the final 10 minutes with a three-goal salvo earning them a 3-1 win and a place in the knockout stage.
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.
"It's a massive improvement on last year. We knew that wasn't good enough."
Smalling last featured for the first-team in May, and asked about his return, the 22-year-old said: "It's been so long and it's been frustrating watching from the sidelines. But it's good to be able to come in and come back with a win as well so I'm feeling happy."
Smalling hailed the clinical finishing of Robin van Persie, whose expertly-taken shot over the stranded Beto was his 11th goal of the season, and the courage shown by the Dutchman's fellow striker Wayne Rooney in stepping up to take a spot-kick only four days after missing one in the Premier League win over Arsenal.
"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."