The France international has highlighted the Barcelona man's weakness ahead of Milan's clash against the Catalan giants in Champions League.
Flamini was full of praise for his former teammate's abilities, but said that the 25-year-old does have a defect in his overall play.
"Fabregas is a player with many qualities, as he has incredible vision and is also very good in front of goal," Flamini told Milan Channel. "As for his defects, at times he 'forgets' to defend."
"Barcelona have great individual talents and play wonderful football, so they are among the favourites for the trophy," he continued.
"Admittedly, we would've preferred to face a different team at this stage, but they will certainly be two fantastic games. We will do everything possible to surprise Barcelona."
Flamini joined Milan from Arsenal in 2008.