Everton say defender Nathan Patterson will be unavailable for up to five weeks after suffering an ankle injury on international duty with Scotland.
The right-back, 20, was forced off during Scotland's 3-0 Nations League win over Ukraine at Hampden Park on 21 September.
Everton say Patterson will undergo treatment on the injury.
The defender has become a regular under Frank Lampard this season and played every minute in the Premier League.
However, he will now miss the club's home game against Manchester United on Sunday 9 October and away to Tottenham six days later.
Everton also play Newcastle, Crystal Palace and Fulham next month, so it remains to be seen whether Patterson will play any part in those fixtures.