Wayne Rooney struck two penalties to earn the Champions League win.
"We started to penetrate more in the second half and luckily we got the two goals," Smalling told MUTV.
"It was great work from Wayne, in and around the box. He and Chicharito were causing problems and with Galati clocking up yellow cards, it was inevitable we would penetrate closer to the goal."
"That was a massive game for us tonight. When we play Galati at home next, if we can get another three points then it will look a lot more rosey than it did a week ago."