Hernandez, who played alongside Casillas during his season-long loan at the Bernabeu last term, hailed the 33-year-old as the best keeper in Real Madrid’s history and wished him all the best for the rest of his career.
Casillas ended his 25-year association with Real Madrid at the weekend, signing a two-year deal with the Portuguese club.
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Hernandez, who has now returned to Old Trafford, expressed his happiness at being able to say he once played alongside the iconic goalkeeper.
"A privilege to be able to say I was a teammate to the best goalkeeper in the history of Real Madrid, @CasillasWorld!!” Chicharito wrote. “Good luck and much success!!".
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Hernandez likely to leave United this summer
Hernandez’s own future is yet to be resolved, although it’s almost certain that the Mexico international has played his final match for the Red Devils.
The 27-year-old failed to convince Van Gaal last summer and did little during his spell with Real Madrid to suggest he should be leading the line for one of the world’s best clubs.
West Ham are understood to have made a formal offer for Hernandez, who is currently playing for Mexico at the CONCACAF Gold Cup in the United States and Canada.
The United States is, of course, where Louis van Gaal has taken a 26-man squad with him for Manchester United’s pre-season friendlies against Club America, San Jose Earthquakes, Barcelona and Paris Saint-Germain.
Manchester United need strikers
Amongst those left behind was Robin van Persie, whose move to Fenerbahce has now been finalised.
With Radamel Falcao also no longer part of Manchester United’s squad - the Colombia international is now on loan with Chelsea - the Red Devils are worryingly short on strikers.
Wayne Rooney is currently Van Gaal’s only senior option, although James Wilson proved last season that he is capable of doing a job upfront if required despite his tender age.
Van Gaal will reportedly hand Memphis Depay a chance to shine in the centre-forward position over the next couple of weeks, while winger Adnan Januzaj could also be tested upfront.
With Hernandez also on his way out, it’s imperative that Van Gaal spends money on at least two new strikers this summer.
Edinson Cavani, Karim Benzema, Harry Kane and Christian Benteke have all been linked with moves to the Red Devils in recent weeks - but no serious offers have yet been forthcoming from the Premier League giants.
Manchester United fans: could Javier Hernandez do a job at Old Trafford next season? Have your say in the comments below!