Valdes arrived at United on a free transfer in 2015 but after just two Premier League appearances, the three-time Champions League winner was quickly banished from the senior team having been accused of refusing to play for the Under-21 side.
The 34-year-old has since revealed that his nightmare spell at Old Trafford almost led him to call it a day, describing his stint at United as the unhappiest time of his club career.
"Sometimes at United I thought 'I don't need this', but I am a fighter and a professional," Valdes told Sky Sports. "This was the biggest fight of my career and I won it.
"It's difficult to explain, but it was hard being alone. The easy way out would have been to end my career."
Given a lifeline by Aitor Karanka at newly-promoted Middlesbrough, Valdes is enjoying his football again, though the 34-year-old insisted he is still a fan of former United and Barca boss Van Gaal.
"I feel like a professional goalkeeper again after all those hard days in Manchester," he added. "But I don't have any bad words for Van Gaal or Manchester United.
"He's like a father figure to me. I don't want to remember the last six months in a bad way - for me he is top. We are still friends and there is no problem between us.
"They helped me through my injury and the manager gave me my chance as a footballer at Barcelona and then again at United.
"I want to say in a public way thank you to the U21 coach Alan Fettis - without him I may have given up football. He made me keep going and stopped me when I was thinking about bringing my career to an end."
After making his Middlesbrough bow in the 1-1 draw at home to Stoke City last week, Valdes is back in action against Sunderland on Sunday.
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