Everton may have to wait until they return to England to fully assess Mikel Arteta's foot injury.
The Spaniard picked up the problem during the 1-0 reverse against Philadelphia Union and has had some preliminary scans.
But David Moyes said he may not know the full extent of the injury until the squad returns to English soil.
Moyes said at a press conference before the game against DC United on Saturday: "Mikel won't play, he has quite a bad injury on his foot. He is out, definitely. We're getting a scan today, we've had an x-ray, we think we might need to go back to diagnose it correctly."
Tim Howard was at the RFK Stadium on Saturday but didn't feature in the game. The USA international had his first training session of pre-season the morning before the match.
Moyes added: "Tim Howard won't play because of his long commitments to the USA national team during the summer months.
"We have had to make sure he has had some time away with his family. He will join the team today and he will be at the game tomorrow. He will have his first training session tomorrow.
"He will be here to say hello but he won't be playing."