Japan coach Hajime Moriyasu has named Mitoma in his final squad for the Asian Cup in Qatar, with the continental championship running from January 12 to February 10.
The 26-year-old has not played for Brighton since limping off during a 1-1 draw with Crystal Palace on December 21 and De Zerbi was unsure whether the forward would recover in time to play in Qatar.
"I'm really surprised because my medical staff told me Mitoma needs four, six weeks for his injury," De Zerbi told reporters on Monday ahead of their trip to face West Ham United.
"For me, it's difficult to think he can play in the Asian Cup. But I am a fan of Mitoma, of all my players, so if they can play for their national team I am very happy and very proud."
Japan are aiming to win the Asian Cup for a record-extending fifth time having previously landed the title in 1992, 2000, 2004 and 2011. They will kick off their campaign on January 14 against Vietnam, before taking on Iraq and Indonesia in Group D.
