Ryan Fredericks will miss West Ham's Europa League home game against Rapid Vienna on Thursday, with fellow right-back Vladimir Coufal also a doubt.
Fredericks was injured in last week's Carabao Cup win at Manchester United, and Coufal - who started Saturday's win at Leeds - also has a problem.
This is West Ham's first home game in Europe, excluding qualifiers, since 2006.
"It's incredibly exciting," said boss David Moyes.
"It's an achievement that the players put an enormous amount of work in to reach. We'll have a huge crowd here for a Europa League group-stage game.