The 54-year-old was the subject of similar comments on the social networking website in September, with the club "strongly" supporting prosecution of those responsible
Barca were humbled 2-0 in Milan in the first leg of the last 16 encounter back in February as Sulley Muntari and Kevin-Prince Boateng goals ensured a comfortable victory.
The return leg saw the Catalans overturn Milan's aggregate lead with an emphatic 4-0 win at Camp Nou, but the memories of the San Siro defeat will spur the Blaugrana on to make sure there is not a repeat result in the Group H match, according to Alexis.
"We learned from last year's experience [2-0 loss against AC Milan in the San Siro]," he told reporters ahead of the game.
"Milan have always shown great heart, it is important to play 90 minutes with great concentration. I always give my 100 per cent."
The Chile international has been criticised from some quarters as they believe he has not reached his full potential since his €26 million move from Udinese.
However, the 24-year-old, who has scored 30 goals in 97 appearances for Barcelona, says detractors spur him on to do even better.
He added: "In Barca there are so many good players. I am ready to play but I do not feel like a starter.
"The criticism gave me more strength. Playing with Neymar gives me happiness and confidence."