The Red Devils lost 2-0 last night in the first leg of their last-16 Europa League tie at Anfield.
Louis van Gaal’s men now face a tough task trying to overturn the deficit in the second leg next week at Old Trafford.
But Smalling claims United couldn’t cope with the pace and intensity of Liverpool.
“We’re disappointed. I think we could have done with getting an away goal – we know we’ve got a difficult job on our hands," Smalling told MUTV.
“They had a lot of runners coming from midfield, and I think our wingers were having to defend a lot, so whenever we won the ball back we couldn’t really sustain it [possession] and keep it.
“In that first half, it was very difficult, we couldn’t get up the pitch.
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"In the second half we were a little bit more comfortable, albeit without creating too many chances.
“We could have really done with getting that away goal.
“I think if you get that goal, you can be a bit more positive, but it’s now up to us to rectify it in front of our own fans. The first goal is going to be crucial.”