El Shaarawy had been out of action since his brief substitute cameo against Cagliari at the start of the month, and has now suffered a fractured foot during a training session in preparation for the Rossoneri's Champions League game at Ajax.
Meanwhile, the team's skipper Riccardo Montolivo, who has not featured for the San Siro side since their 2-2 Serie A draw against Torino on September 14, could make Tuesday's Champions League clash with Ajax after resuming regular training at the weekend.
However, Allegri has stressed that he may need to use Montolivo sparingly ahead of the vital clash with champions Juventus on Sunday.
"I'll have to manage them as best I can because we’ll be up against Juventus after the game versus Ajax," Allegri told reporters.
Milan got their Champions League campaign under way with a win 2-0 win over Celtic in their first game.