The Premier League champions' stuttering domestic form continued at the weekend as they drew 1-1 with Southampton, but United sit joint-top of their Champions League group on four points.
"We're disappointed, yes. Our own performance hasn't been up to scratch, especially in possession," Evans told reporters at a press conference.
"We've been quite sloppy and we haven't been controlling games as well as we would have liked. I think Southampton probably controlled the game a bit better than we did.
"The onus is on us. We know we haven't played well enough as individuals or as a team. We want to make sure we raise our performance and produce better performances."
Sociedad are yet to record a Champions League point thus far this season.