The Premier League giants were beaten 1-0 by the Eredivisie side in their first Group A fixture at De Kuip in Rotterdam on Thursday night.
Tonny Vilhena's 79th minute goal did the damage on a night where United were expected to come away with maximum points, prompting the following reaction from stand-in skipper Smalling.
"I think we came into this game very confident and we thought we'd get a good result," Smalling told MUTV.
"We wanted to give ourselves a good start to the group but unfortunately we didn't play up to our usual standards.
"They didn't create too many chances but we didn't do enough either to win the game. I think our urgency was there in the second half but we needed to have that in the first half as well."
The negative result was United's second on the bounce in the wake of Saturday's 2-1 derby defeat to Manchester City and Smalling is now urging a return to form in Sunday's Premier League meeting with Watford at Vicarage Road.
"We've got to show a lot of character because it's unacceptable to go and lose three games in a row," he added.