Brown, who won five Premier League titles and two Champions League winners' medals during his time at United, believes Harry Maguire and his defensive colleagues need to step hip to the mark in the second half of the season.
"The concentration levels of some of the defensive lads have not been acceptable at times," Brown told the Sunday World.
"There have been a lot of errors. They have conceded a lot of silly goals. The goal they conceded in Istanbul when there wasn't a defender in sight, it wasn't good enough. There was a lack of communication there.
"They have let in really weak goals at times and a lot of it has come down to individual errors and at a club like Manchester United, it's not good enough.
"Consistency is the issue at the moment. They are up and down too much. On any given day, when they are at their best, they are capable of beating most teams.
"Their home form in the league hasn't been acceptable at times, but I think they have made some progress under Ole.
"From where he was a month ago to where he is now, we are more comfortable, but we need to get to a point where the consistency is there. After Christmas, you start to know where you are in the league and he will be judged then."