Macheda replaces Cassano at Sampdoria
Samp have been badly short of firepower since Antonio Cassano, whose move to AC Milan was officially registered with Serie A on Monday, was suspended in October for verbally abusing club president Riccardo Garrone.
"Macheda has easily filled the void left by Cassano, he will definitely do well," Garrone told reporters.
"In these five months we will try to treat him well. With him here, the enthusiasm has increased tenfold."
United manager Sir Alex Ferguson, who has seen fellow forward Danny Welbeck enjoy a successful loan spell at Sunderland, said he wanted Macheda to gain the same first-team experience having been on the periphery of the squad this term.
Several clubs in England and Italy were interested in the former Lazio trainee but the chance of fighting for an immediate first-team place alongside Samp's Giampaolo Pazzini is likely to have swayed the 19-year-old's decision.