So far this season, their form has been erratic as ever with only three wins in eight Serie A matches, and the injuries have continued to pile up. On Tuesday, they will be hoping to vary the routine by beating the Catalans.
The two sides have met six times in the last two seasons with Barcelona predictably dominating. They have knocked seven-times champions Milan out of the Champions League twice in a row, in the quarter-finals two seasons ago and in the round of 16 last term, and enjoyed a win and a draw in two group stage meetings in 2011/12.
Milan's only win in those six matches was in the first leg of last season's round of 16 clash, but they were overwhelmed 4-0 in the return.
With Barcelona yet to concede a goal or drop a point in Group H, Milan clearly have a tall order although, whatever happens on Tuesday, both sides should comfortably progress, Ajax Amsterdam and Celtic having so far managed only one point between them.
For the second season in a row, Milan, who seem to be stuck in an interminable transitional phase, have made a slow start. They have taken 11 points from eight games in Serie A and have already dropped 13 points behind leaders AS Roma, suggesting that once again they will have to focus on next season's Champions League rather than challenging for the title..
Most of their points have been won through sheer grit and determination rather well-polished performances.
Remarkably, nine of Milan's last 13 goals have come in the last 10 minutes of their games, including all three they have scored in the Champions League.
Allegri's side scored both goals in the 2-0 win over Celtic in the dying minutes and needed a stoppage time penalty to draw at Ajax.
In the league, they scored twice in the last five minutes to draw 2-2 at Torino and repeated the trick in a 3-3 draw at Bologna, where they trailed 3-1 until the 89th minute.
Most frustrating of all has been the continued injury crisis with goalkeeper Christian Abbiati, defender Daniele Bonera and Mattia De Sciglio and forwards Stephan El Shaarawy, Mario Balotelli and Giampaolo Pazzini all in the treatment room.
El Shaarawy has not played since Sept. 1 after suffering first a thigh injury and then a microfracture of his foot, Pazzini has been out since May with a knee injury and Bonera since July.
Balotelli missed Saturday's 1-0 win over Udinese with a thigh muscle injury he suffered during Italy's World Cup qualifier against Armenia and is doubtful for Tuesday.
Last month, former Milan forward Alexandre Pato, who suffered an astonishing sequence of muscular injuries at the club, questioned the treatment at Milan.
"During one-and-a-half years there, I played, I got injured, I was out for one month, I got injured again... Since I came back to Brazil, I haven't had any more injuries," said Pato, who joined Corinthians in January
"The treatment there is different," he said in a programme on the Brazilian cable channel Sportv. "While are you out of action, you do some work in the swimming pool and a little bit of physiotherapy. Then in one week they make you put in a lot of effort.
"I think it all shows that what happened to me at Milan had nothing to do with me, as other players are still getting injured. I was not to blame."
Allegri replied at the time that Pato should have a sense of self-criticism instead of making "gratuitous accusations."
Barcelona dropped their first league points of the season in a 0-0 draw at Osasuna on Saturday, when Lionel Messi came on in the second half for his first appearance since suffering a thigh injury at the end of last month.