Japan international Miyaichi, 20, suffered an ankle injury after coming on as a substitute in Wigan's 3-0 shock quarter-final win at Everton on Saturday which put the Premier League strugglers into the last four for the first time in their 81-year history.
Wigan said on their official Twitter feed that they were still awaiting the result of scans on the Japan international who joined the Latics on a season-long loan from Arsenal but has made just seven appearances, six as a substitute.
The youngster is now set to miss Wigan's semi-final against either Millwall or Blackburn Rovers at Wembley on the weekend of April 13-14 as well as the team's relegation run-in.
Miyaichi's injury means that if Manchester United, who play Chelsea in a sixth round replay, make it through to the final on May 11, their midfielder Shinji Kagawa could become the first Japanese to play in the final.