The Ecuador international was initially signed to replace Cristiano Ronaldo on the right wing.
However, he has since transformed himself into one of the Premier League’s most consistent right-backs.
Valencia made the transition when Van Gaal was in charge.
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Antonio Valencia has revealed he feared for his Manchester United future
Antonio Valencia has turned into one of Manchester United's best performers
And the 31-year-old has admitted he feared the Dutchman would sell him following the arrival of wingers Angel Di Maria and Memphis Depay in 2014 and 2015 respectively.
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“I am very happy and I'm very grateful for the manager's words,” he told the Daily Mail.
“I don't believe I'm the best but I am really pleased to still be here.
“There was a time when I saw players of the class of Angel Di Maria and Memphis Depay arriving and wondered if my time was reaching an end. Perhaps I needed to start looking for a new club.
Manchester United shelled out big money to sign Angel Di Maria in 2014
“Thankfully I just concentrated and trained hard and maybe that's why I'm still here.
“Mourinho has shown faith in me to stay here so I'm delighted. I don't want to say I am 100 per cent a right back. I still feel like a right-sided attacking player but I guess I am getting there.”
Valencia is expected to start when United face Watford this afternoon (3pm).
The Red Devils are aiming to secure back-to-back Premier League wins after thumping Leicester 3-0 on Sunday.