The Hammers host Romanian club Astra Giurgiu in the first leg on Thursday but Reid will be absent in central defence after missing last week's crucial penalty shootout win over Maltese side Birkirkara FC.
New Zealand international Reid was absent with a minor injury and is expected to be on the sidelines again when Asytra come to Upton Park according to co-chairman David Gold.
Gold was responding to a fan Tweet regarding the fitness of Reid.
Winston will no be risked on Thursday. dg https://t.co/m3iNBJDMYA
— David Gold (@davidgold) July 28, 2015