Peter Schmeichel was disappointed Manchester United didn't put a poor Real Sociedad team to the sword on Wednesday night.
David Moyes' men were ahead after just two minutes thanks to an own goal from Sociedad defender Inigo Martinez and maintained their advantage to retain their place at the top Group A, a point clear of Bayer Leverkusen.
However, despite securing the crucial three points, former United keeper Schmeichel feels his former side should be doing more to finish off opponents and fears their inability to do so hints at more significant, underlying problems.
"It was a good start and they've done that for the third game running now, but again we saw some of the problems which have been surfacing this season," said the 1999 Champions League winner.
"For some reason United stopped playing. I don't understand why the players are doing that but they are letting teams back into the game.
"They should have gone on and killed the game off but it looks as though they don't have the ability to do that.
"Real Sociedad were there for the taking, they weren't a great team and didn't defend well.
"The way Man United performed today, to win 1-0 with an own goal, is a bit disappointing. But at the end of the day Manchester United get three points and put themselves in the driving seat for the group and it's down to them now to go on and qualify."
Graeme Souness, however, was underwhelmed by United's performance.
The Old Trafford outfit went with Antonio Valencia, Michael Carrick, Ryan Giggs and Shinji Kagawa in midfield, but an unimpressed Souness warned Moyes he must find a better solution in that area when United face stronger sides later in the competition.
"United started so well and then after scoring so early you expect them to find another gear and really boss the game and see it over and done with by half-time. But they didn't do that," he told Sky Sports.
"They gave the ball away too easily, gave it away without being under any real pressure. The balance in midfield, when they play better teams, will get found out, too.
"They can't go that way against better teams - that was a poor team they were playing tonight."
United's solitary goal sparked some controversy and there was disagreement in the Sky Sports studio as to whether Javier Hernandez, who was standing in an offside position when Wayne Rooney fired in the shot which led to Martinez's own goal, was interfering with play.
Jamie Carragher was convinced Martinez's ugly, mis-timed clearance into his own net was caused by Hernandez's presence.
"It is a difficult one but, for me, I cannot believe a professional footballer would have kneed that - the only thing I can think of is that he is nervous with the presence of Hernandez and rushes it."
Schmeichel, however, disagreed: "I think it is completely down to poor defending," said the former keeper.
"Martinez is a young player and it's rash. I don't think Hernandez interferes in any shape or form. For me it's really poor and if Hernandez was not there the same thing would have happened."