"It feels like we lost the game," Smalling said on the United website after Wednesday's game left the last 16 tie in the balance, with the English side short of an away goal to take back to the return at Old Trafford on March 15.
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"Everyone in the dressing room was a bit dejected.
"It just didn't quite click; our passing was a bit off throughout the team. We didn't test their goalkeeper enough."
Smalling, who joined from Fulham last year and is deputising for the injured Rio Ferdinand at centre-back, received good reviews for a performance that summed up the United display as a whole - rock solid and entirely unspectacular.
Midfielder Michael Carrick echoed his 21-year-old teammate's evaluation of the match in which the Premier League leaders lacked an attacking spark.
"We're pretty disappointed with the performance, it just didn't happen for us," Carrick said.
"Our passing wasn't great so we didn't give ourselves the chance of creating chances in the game.
"The clean sheet gives us a good chance of winning the tie at home. We're confident we can do that but we'll have to play better than tonight. The other side of our game - our attacking and possession as a team - has got to be better."
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