Rooney can see United finishing fifth in the Premier League this season.
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He told the BBC: "I think they still need one or two more signings but I can see what Amorim is doing.
"It's looking better for Man United and I think they will sneak into the top five. I hope they do and you never know."
Meanwhile Rooney, who will act as a pundit with the BBC this season, rates champions Liverpool as the team to beat.
He also states Jeremie Frimpong will prove the signing of the year, with Reds teammate Rio Ngumoha being the league's breakout player.
In terms of which of the three promoted teams will most impress, Rooney tipped Leeds to shine ahead of Burnley and Sunderland.