The Spain international, in his 12-word tweet, also told the Mexico international not to forget that he’s the best.
De Gea posted a collage of the two of them together, alongside the caption: “Happy birthday brother!! You're the best, don't ever forget. Love u Bro @CH14_”.
It’s clear that De Gea and Hernandez struck up an excellent bond during their time working together at Manchester United.
At the start of the season, shortly after Hernandez had sealed a season-long loan to the Bernabeu, De Gea told Spanish newspaper Marca that the live-wire striker is one of the best people he has ever met in football.
"He is a great person – one of the best people I have met in football," De Gea was quoted as telling Marca by FourFourTwo. "He will do very good things in Madrid.
"He doesn't play very often but he scores a lot of goals."
Hernandez to return to United - before being sold
Hernandez, 27, ended up scoring nine goals in 33 appearances for Real Madrid and will shortly return to Old Trafford after Los Blancos passed on the opportunity to turn his loan deal into a permanent move.
It’s expected that United boss Louis van Gaal will attempt to find a buyer for the former Chivas Guadalajara star, who arrived at Old Trafford back in 2010, this summer.
That Hernandez will be used as bait to sign one of Manchester United’s top transfer targets is not beyond the realms of possibility.
He could find himself at Southampton this summer as the Red Devils attempt to strike a deal for defensive midfielder Morgan Schneiderlin; it’s reported that Ronald Koeman would like to bring Chicharito to St Mary’s.
Real Madrid desperate to sign De Gea
While Hernandez returns to Old Trafford, De Gea is expected to seal a move in the opposite direction.
Real Madrid are desperate to add the world-class goalkeeper to their ranks in time for the start of the 2015-16 campaign, but must convince Manchester United to sell this summer.
United, if they fail to strike an agreement with the reigning European champions, run the risk of losing De Gea on a free transfer when his contract expires this time next year.
Should Louis van Gaal give Javier Hernandez another chance at Manchester United? Have your say in the comments below!