The Red Devils pair enjoyed a breakthrough season in the Premier League under Louis van Gaal, although the Dutchman has since been sacked.
The France international scored 11 goals in 31 top-flight games for the 20-time English champions following his £36m move from Ligue 1 side Monaco.
Fosu-Mensah was used sparingly in the second half of the season as the Dutch defender racked up eight Premier League appearances.
And the teenager took to Instagram to post a photo of him embracing Martial and wrote the following caption for the French forward:
United finished in fifth place in the Premier League table behind Manchester City on goal difference.
The Red Devils won the FA Cup thanks to a 2-1 extra-time victory over Crystal Palace at Wembley.
Jose Mourinho was appointed as the new United manager on Friday.
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