De Gea finished the campaign as Atletico's first-choice keeper and has not looked back since. He signed for United two seasons later, in 2011, and is widely recognised as being one of the best keepers in the world.
"I gave David his debut (at Atletico against Porto in the Champions League in September 2009)," Sanchez Flores told reporters at his pre-match news conference.
"He was the third-choice keeper and after one month he was first choice.
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"He was amazing. You just need to focus on the training and watch how he is taking the ball and handling it. For me, he is probably the best goalkeeper in the world."
The 50-year-old's current keeper at Watford, Heurelho Gomes, is not in the same class as De Gea but Sanchez Flores backed him to recover from a dreadful display in Watford's 2-1 loss to Leicester City on Nov. 7.
A Gomes error handed the Foxes the opening goal and the Brazilian then brought down Jamie Vardy in the box, conceding a penalty from which the in-form Leicester striker slotted home.
"He (Gomez) knows accidents are possible in football.
"I said to him when we were coming back from London: 'Heurelho, I love you more than before'. We love him," Sanchez Flores said.
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by John O'Brien)
