Another bad day for Bournemouth pushed the Cherries closer to a return to the Championship as they remain rooted in the bottom three despite taking a shock lead at Old Trafford through Junior Stanislas.
Teenage sensation Greenwood, Rashford from the penalty spot and Martial hit back before half-time.
Former United forward Josh King’s penalty briefly gave the visitors hope early in the second half, but another thumping finish from Greenwood and a trademark Fernandes free-kick gave United five goals at Old Trafford in a Premier League game for the first time since 2011.
Victory takes Ole Gunnar Solskjaer’s men a point above Chelsea into fourth as they extended an unbeaten run stretching back to January to 16 games.
United had kept 11 clean sheets in their previous 15 matches without defeat, but defensive lapses were the only blight on another satisfying afternoon for Solskjaer.
Harry Maguire and David de Gea were at fault for the only points the Red Devils have dropped since the Premier League’s restart in a 1-1 draw at Tottenham.
Both were culpable as Stanislas nutmegged the world’s most expensive defender and then fired in at De Gea’s near post completely against the run of play on 16 minutes.
Those errors were not as costly this time as the blossoming front three of Greenwood, Martial and Rashford quickly turned the game around.
Greenwood’s star is rising fast and the 18-year-old blasted home from Fernandes’s pass to get United back on track.
The hosts were then gifted a second by Adam Smith’s handball inside the area and Fernandes stepped aside to let Rashford net for the first time since January.
Martial completed the set with a stunning strike from outside the box to reach 20 goals in a season for the first time in his career.
The game appeared to be over at the break, but a sloppy start from United presented Bournemouth with a route back into the game.
Eric Bailly replaced Victor Lindelof at half-time and the Ivorian handled as he tried to control a loose pass from Nemanja Matic.
King, who was a United target in January as Solskjaer desperately looked for cover for the then-injured Rashford, dispatched the penalty and moments later Bournemouth had the ball in the net again only for Arnaut Danjuma to be flagged offside.
As in the first half, United’s response was quick and emphatic. Greenwood smashed home his 15th goal of the season on his supposedly weaker right foot to restore a two-goal lead.
The signing of Fernandes has been the catalyst to United’s turnaround before and after a three-month stoppage due to the coronavirus lockdown and the Portuguese midfielder took his tally to seven goals in 14 games by hooking a free-kick in at Aaron Ramsdale’s near post.
Rashford also had another goal ruled out for offside, but United were happy to settle for five as they moved into the top four for the first time since September.
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