United had not long fallen behind to Alan's 49th-minute penalty when some of the lights went out at the Estadio AXA, which led to play being stopped for around 15 minutes.
The visitors had struggled to fashion much from the possession they had had up to that point, and the improvement in their game after the unscheduled interval suggested Red Devils boss Alex Ferguson had made the most of the opportunity to deliver an additional team-talk.
They resumed with Rio Ferdinand having come on for Jonny Evans and following further substitutions to bring Robin van Persie and Rafael into the fray, United turned the contest on its head.
Van Persie guided in an 80th-minute effort from outside the box, Wayne Rooney converted a penalty five minutes later and Javier Hernandez added a close-range strike in stoppage-time as Ferguson's men registered their fourth win in four Group H games to confirm their qualification for the last 16 as group winners.
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."
Smalling hailed the clinical finishing of 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 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."
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."
Having collected the maximum 12 points on offer so far, United can go into their final two Group H fixtures knowing they cannot be caught by Galatasaray or Cluj, both on four points, or Braga, who have three.
It has been a considerable return to form for the Red Devils after they failed to get out of their group last season, and means that for the time being, their focus can be concentrated on the Premier League, which they currently lead.
Smalling said: "It's a massive improvement on last year. We knew that wasn't good enough."