Sweden great Ibrahimovic, who on Tuesday confirmed he is to come out of international retirement, suffered a thigh injury in the 2-1 Serie A win at Roma on February 28 and has not featured since.
It meant Ibrahimovic missed the Rossoneri's Europa League last-16 first-leg tie against his former club at Old Trafford last week, in which Milan secured a useful away goal in a 1-1 draw.
However, Ibrahimovic has trained for an injury-hit Milan this week and is in line to play a part in a crucial second leg at San Siro.
Ismael Bennacer is also back involved but head coach Pioli still has several other issues to contend with.
"Ibra and Bennacer are recovered, they are important for us and they are among the squad," Pioli said.
"For the others, [Davide] Calabria, [Rafael] Leao, [Alessio] Romagnoli and [Ante] Rebic, we have to wait for tomorrow morning. We'll take a few more hours to decide.
"His [Ibrahimovic's] return is important, he will not be able to have 90 minutes in his legs and tomorrow we will see what to do."
Ibrahimovic has 16 goals across all competitions this season and in United faces a team with who he enjoyed EFL Cup and Europa League success under Jose Mourinho – although the latter final he missed out on due to a serious knee injury.
United now have Ibrahimovic's former Paris Saint-Germain team-mate Edinson Cavani among their ranks, a player who claimed the Swede's status as the club's all-time leading scorer.
Asked which striker is better, Pioli diplomatically replied: "Ibra is a champion. I have never coached Cavani, he is a great player, but I hold on to Ibra."