The defender's close-range effort was shown to have crossed the line by a matter of inches after Tigers' goalkeeper Allan McGregor had fumbled the ball in a goalmouth scramble.
Goalline technology showed referee Anthony Taylor that Smalling had indeed scored, his first strike since his goal against the same opposition last Boxing Day, but the 25-year-old told MUTV he wasn't sure Saturday's would be given.
"I didn't think it was over the line initially," he admitted. "Thankfully the goalline technology gave it. I think [if] it's a case of whether it's over or not, the [technology] takes away that doubt."
The Reds' third win in a row came from a confident display that pleased the team and manager Louis van Gaal alike in the wake of a battling performance at Arsenal one week ago.
"The manager said it was our best game of the season and I think it was," Smalling added. "It was a complete performance. I think he said of the Arsenal result that we ground that out without particularly playing well, but this time we did. He's said he wants six points and we want to deliver that on Tuesday against Stoke."
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