The Colombia international struggled to make an impact at Old Trafford under Louis van Gaal after equally failing to hold down a regular spot at Chelsea in the Premier League the season before.
Falcao, who turned 30 on Friday, has since returned to Monaco after his loan spells in England and has been in fine form for the French side, scoring 14 goals and making two assists in Ligue 1 so far this term.
And De Gea used his personal Instagram account to send a supportive message to his former team-mate as he celebrated his 30th birthday.
De Gea posted the picture above on the app and wrote the caption: “Happy birthday @falcao a crack both in and outside the pitch. We miss you! 🐅”
Falcao endured a miserable spell at Chelsea in the 2015-16 season, as he managed to score just one goal in 12 appearances in all competitions.
The striker had only managed four strikes for Manchester United in the season before.
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