Simon Rolfes insists Bayer Leverkusen must improve defensively if the club are to progress in the UEFA Champions League.
Leverkusen started their Group A campaign with a 4-2 defeat at English champions Manchester United on Tuesday.
A brace from Wayne Rooney - which took the striker to 200 goals for United - and strikes from Robin van Persie and Antonio Valencia left Leverkusen deflated at Old Trafford.
Sami Hyypia's side were for the most part outplayed by a fluent United, but showed signs that they can pose a threat with goals from captain Rolfes and Omer Toprak.
But midfielder Rolfes believes Leverkusen failed to play to their full potential and urged his team-mates to improve ahead of the visit of Real Sociedad next month.
"We can play a lot better than we did," he said.
"We played well in parts, but we didn't manage to maintain that level of performance throughout the 90 minutes.
"Had we been able to do so, we may have caused Manchester United a few more problems.
"We were in with a shout at 2-1, but we brought the third goal upon ourselves. We can't afford mistakes like that."comments