Fredericks could miss the rest of the season after damaging shoulder ligaments.
Hammers boss Moyes is now waiting for a second medical opinion on the extent of the problem.
Ahead of Monday's trip to Liverpool, Moyes said: “We're doing everything we can to get him back but I couldn't confirm if he will need surgery, or just healing time.
“We are just waiting on the advice. You never want to play any players who are not fit or injured and I won't do that."