The Red Devils suffered a big setback in their bid to win a first Europa League title after a 2-0 loss to Liverpool at Anfield on Thursday night.
Daniel Sturridge broke the deadlock from the spot before Roberto Firmino doubled Liverpool’s lead in the first leg of their last-16 clash.
West Ham, on the other hand, came from 2-0 down to beat Everton 3-2 in the Premier League last weekend and hoisted themselves above United and into fifth place in the table.
And former Aston Villa and Everton forward Gray is confident the Hammers can secure a memorable FA Cup victory at the Theatre of Dreams.
“I’d be all over West Ham, I think there are a lot of good things happening at the Hammers at the moment,” Gray told Paddy Power’s blog.
“They’re playing with supreme confidence, their big players are influencing games. They’re winning matches, they’re tough to beat and I think they’re going to take all of that confidence in to the weekend and I see them giving United a really torrid time on Sunday afternoon. I fancy them strongly.
“They’ve won their last four games including their FA Cup tie and the players will fancy it big time against United.”
Enhanced odds: Get 7/1 on Man Utd to beat West Ham
West Ham are in fifth place and two points above Louis van Gaal’s struggling Red Devils side ahead of Sunday’s trip to Stamford Bridge.
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