The Dutchman gave the visitors the lead before Liverpool stormed back to win 3-1.
But Willems has taken a few positives from how his side played in spells against the champions of Europe.
“We need to believe and keep working, trying," he said.
"We are capable. We have had tough games already against Arsenal and lost also because of a mistake. We beat Tottenham and played well. We need to keep believing."