Andy Carroll and Winston Reid will both be sidelined by injury for at least three months, West Ham have confirmed.
The key pair suffered setbacks in their recoveries from respective ankle and knee problems during the club's pre-season training camp in Switzerland.
They have returned to London ahead of schedule and will undergo operations.
The setback comes as particular blow to Carroll, who is entering the final year of his deal at London Stadium.
Manager Manuel Pellegrini confirmed the Hammers will remain active in the transfer market in search of reinforcements, with Felipe Anderson reportedly on the verge of joining from Lazio.