Brighton was in full control and defeated the table toppers 3–0.
But in the 58th minute, with a 2–0 lead, the heavy setback came.
"It didn't look good, but I'm positive. It's definitely the back of the thigh," said coach Fabian Hürzeler at the press conference after the match.
The injury comes at the worst possible time. With just one month left until the World Cup, and Japan in the same group as Sweden, there is a risk Mitoma will miss the championship.
Mitoma has nine goals in 31 international appearances. Japan will face Sweden on June 26 at 1:00 AM.
