Three days before the Europa League final against Ajax, Jose Mourinho made nine changes on his 250th game as a manager in England’s top flight, with Demetri Mitchell, Harrop and substitute Angel Gomes given United debuts.
And United impressed in a first-half performance that proved enough to secure a comfortable victory prior to their season-defining trip to Stockholm.
Harrop relished opening the scoring with a fine finish after being set up by Paul Pogba, who then quickly doubled United’s lead on his return from compassionate leave following the death of his father.
Pogba and Jesse Lingard were both substituted before half-time as Mourinho took no chances ahead of an Ajax match that could give them a second major trophy of the season and seal a place in next season’s Champions League.
Palace edged a quiet second half, Christian Benteke hitting the post with a header as they failed to find a way back into the game.
The hosts finish the season sixth in the table on 69 points, while Palace drop to 14th on 41.