Milan are without a win in their last four matches in all competitions and sit 11th in Serie A following a 2-0 defeat to Fiorentina on Saturday.
However, Massimiliano Allegri's men have not yet lost in the Group H and would overtake the Spanish champions at the top of the pool with a win at Camp Nou on Wednesday.
And Argentina international Mascherano is under no illusion as to the threat Milan pose.
"They are a club of status, tradition and class," he told Extra Time.
"They have not had a good start to the season, but in their games the fact is that they have demonstrated something.
"Not only against us but also against Juventus. They may have lost 3-2 against the latter, but they played an interesting game."
Brazilian midfielder Kaka was one of the standout performers for Milan when Barcelona drew 1-1 at San Siro last month.
And Mascherano has not been taken aback by how the former Real Madrid playmaker has coped with his return to Italy, despite having initially seen his introduction into Allegri's team delayed because of injury.
"I am not surprised by his improvement because I believe that football is 80 per cent psychological and 20 per cent physical," he continued.
"Kaka has experienced with Milan his prime as a player (so) it is not possible to feel better anywhere else.
"I do not know if he will return to the best Kaka, but surely we will see a great Kaka, like the one we faced at San Siro two weeks ago."
