Tomkins suffers dislocated shoulder
Tomkins is reported to have sustained the injury during training on Thursday, with the club confirming he later received treatment at hospital.
No timeframe has been put on a return for Tomkins, but he could be ruled out for the rest of the season.
The news is a further blow to manager Sam Allardyce, who is already without fellow centre-back Winston Reid for Saturday's Premier League trip to Arsenal.
"West Ham United can confirm that James Tomkins has suffered a dislocated shoulder," the club posted on their official Twitter account.
"The 25-year-old was taken for hospital treatment and will continue to be assessed by the club's medical staff."