El Shaarawy was set to return to action in Tuesday's UEFA Champions League encounter at Ajax after being out for one month.
However, the 20-year-old suffered a microfracture of the fourth metatarsal in his left foot and could be out for up to a month to add to Milan's injury woes.
"I cannot believe it!!!!" El Shaarawy said on Twitter.
AC Milan coach Massimiliano Allegri was disappointed with the news.
"He could be out between 20 days to a month," Allegri said on the club's official website. "It is a real shame because he was in good condition having recovered from a muscular injury."
Milan are already without defenders Ignazio Abate, Daniele Bonera, Matias Silvestre and Mattia De Sciglio as well as midfielders Andrea Poli and Kaka, all of whom have picked up injuries in recent weeks.
To complicate matters for Allegri, the team's top scorer Mario Balotelli will miss Sunday's match at Juventus as he must serve his third and final match-ban.