And Mikel Arteta is set to be another high-profile absentee for the Toffees' Premier League opener against Tottenham on Saturday - assuming it goes ahead following three days of rioting in the capital.
Republic of Ireland international Coleman was forced off by a bad challenge from Carlos Marchena during Friday's friendly against Villarreal.
X-rays confirmed he had not fractured the joint but further scans have revealed the ligament damage.
Head physio Danny Donachie told the club's website: "Seamus has torn the lateral ligament in his right ankle. He will be visiting a specialist in London later this week for further assessment."
No timescale has been put on the 22-year-old's recovery.
Coleman, named Everton's Young Player of the Year last season, was already suspended for the opening two games of the new campaign after a red card against Chelsea on the final day of last season.
The first of those, at White Hart Lane, is one of a trio of London fixtures of particular interest as authorities consider whether the Premier League's opening weekend will be affected by the rioting sweeping the country.
Should the game proceed as planned, Arteta will join Coleman on the sidelines as a result of a foot injury which has kept him out of much of the club's pre-season programme.