After going down in the friendly defeat to Sigma Olomouc on Tuesday the winger was stretchered off in a leg brace. Scans have since revealed that he has ruptured his achilles tendon and could be out until December.
The player has since flown to Strasborg for surgery. West Ham's head physiotherapist George Cooper spoke to whufc.com.
"A major shock"
"The injury was completely out of the blue," said Cooper, "Julien has no history of any achilles problems and this kind of rupture is usually predisposed by past steroid injections in the area - something he has never had - so it is a major shock.
"He was already planning his rehabilitation in the taxi on the way to the hospital and we will be working with him every step of the way to ensure he returns to full fitness as soon as possible."
"Devastating blow"
Alan Curbishley spoke of his disappointment, a feeling no doubt matched by the majoritity of Hammers fans who will have been desperately excited at the prospect of seeing the pacy winger ply his trade at Upton Park.
"It's a devastating blow for everyone, especially Julien.
"He had fitted in so well and was showing all the attributes needed to be a success in the Premiership.
"You could see in the short bit of football he has played with us that he was going to be a big hit with the supporters - he is strong, fast, gets up and down, and understands the game - so to lose him after an hour of football and just two weeks at the club is a big disappointment.
Christmas time
"Hopefully we will see him back in action around Christmas time but the main thing is that he follows his rehabilitation programme and only returns when he is 100% ready.
"We wish Julien a speedy recovery and, having got to know the boy over the past week or so, I am certain that he will respond to this setback in the right way."