The Everton striker forced the Japanese winger, on loan from Arsenal, off on a stretcher during the Latics' 3-0 FA Cup quarter-final victory with the damage likely to keep him sidelined for the remainder of the season.
"It was a terrible challenge. We are waiting on the scan but we are not very positive on it," he said.
"We think it will need a long recovery period and maybe he will even need surgery.
"We need to be a little bit expecting the bad news.
"He is very fortunate he has not got a broken leg because of the nature of the challenge.
"We feel he ruptured the ligaments. We will have to wait and see but he doesn't seem to have an opportunity to finish the season."