Although the Argentinean is still to be given the final 'OK' by the club's medical staff, he has recovered almost a week faster than expected from the broken radial bone he suffered against Sevilla on October 20th.
Barely two weeks have passed since Messi's injury and he was originally given a three-week recovery period, however, he was back in training with his team-mates at the end of last week.
When asked about an early return for Messi, Barca coach Ernesto Valverde on Friday insisted he was not going to take any risks with the Argentinean and he was duly left out of the side which won 3-2 away to Rayo Vallecano on Saturday.
Despite worries they were dependent on their star player Barca have managed well without Messi, beating Inter 2-0 in the Camp Nou, before thrashing Real Madrid in the 'Clasico'.
They also beat Cultural Leonesa 1-0 in the Copa del Rey before Saturday's win in Vallecas and consequently it would not be a shock if Valverde decides not to risk Messi and instead reserve him for Barca's Liga Santander game at home to Betis on November 11th.
Barca currently have nine points from their three Champions League group games and a win in Milan would assure their place in the knockout stage of the competition.
Tuesday also sees Atletico Madrid look to avenge the 4-0 thrashing they suffered away to Borussia Dortmund a fortnight ago.
Atletico go into the game after a disappointing 1-1 draw away to Leganes in which they again struggled to create clear-cut chances from open play, while Dortmund travel to Madrid as leaders of the Bundesliga following a 1-0 win away to Wolfsburg.
Spain international striker, Paco Alcacer, will be able to play for the Germans on Tuesday after a minor muscle problem sidelined him from the first meeting between the two sides.