Bellion broke his leg in May and is at least a month away from full fitness but the West Ham manager Alan Pardew is confident that the 22-year-old Frenchman can help the Premiership newcomers stay in the top flight this season.
Pardew, whose search for a striker will go on despite Bellion's arrival, said: "He has been troubled with injuries over the last two seasons but we are satisfied they are all cleared up now. Outside Manchester United we consider we have the best backroom staff in the game, so I am confident we can give him all the support he needs."
West Ham began life back in the Premiership with a convincing 3-1 defeat of Blackburn on Saturday and the club's hopes of a bright new dawn will have been further boosted by the comments of Kia Joorabchian, the Iranian-born head of London-based consortium Media Sports Investment, who gave notice of a bid for the Hammers earlier this month.
Joorabchian yesterday said he had not made a bid yet, but it may be only a matter of time. "My West Ham project has to be worth around £200m," he said. "That's £45m on buying the club, £30m on its debts, another £30m on keeping the cash-flow going and around £100m on new players.
"There is no point in buying West Ham unless you are going to turn them into one of the top teams in the country. We are going to make a bid, although at the moment we have not done anything. There is still a possibility it could be another Premiership club other than West Ham."
West Ham's chief executive Paul Aldridge says that, as a plc, the club are duty-bound to notify their shareholders if they receive a realistic bid but, up until now, that has not happened.
The Manchester United manager Sir Alex Ferguson believes that Bellion's move to Upton Park will help him improve.
Bellion made only 40 first-team appearances at United after they bought him from Sunderland for £2m.
Ferguson said: "He needs match experience. This will be very beneficial to him and I'm sure he will come back a better player."