Mitoma was forced to miss the Seagulls' 1-1 draw with Everton in the Premier League last weekend, but his problem was unrelated to the nasty challenge from Sheffield United's Mason Holgate a week earlier.
De Zerbi has since confirmed that the 26-year-old is nursing a back injury which is set to rule him out for at least a couple of months.
Speaking at a press conference ahead of Brighton's FA Cup fifth-round tie with Wolverhampton Wanderers on Wednesday, De Zerbi said: «A bad, bad situation. I think we've lost Mitoma until the end of the season.
»He has a back injury, it is an important problem. I think he will be out for two or three months and two or three months finishes the season."
Mitoma previously missed three Premier League games between late December and early January due to an ankle injury, and the Japan international subsequently emerged as a doubt for the Asian Cup.
However, Mitoma was fit to feature in Japan's last-16 and quarter-final fixtures before returning to Brighton earlier this month.
The winger is now dealing with a fresh injury setback at a time when Brighton are still competing in three different competitions – the Premier League, FA Cup and Europa League.
Brighton still have 12 games remaining in the Premier League and currently sit seventh in the table, five points behind Manchester United in sixth and eight points adrift of Tottenham Hotspur in fourth.
The Seagulls are also gearing up for a challenging Europa League last-16 tie against Roma, with the first leg in Italy taking place on March 7.
Mitoma has been a standout performer for Brighton since
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