The former Barcelona keeper signed a two year deal at Middlesbrough this summer.
Victor Valdes was close to quitting football
Valdes joined United in January 2015 after the club agreed to help with his rehabilitation from a serious knee injury sustained at former club Barcelona.
But his widely publicised move turned sour when he was frozen out last summer despite speculation surrounding first choice keeper David de Gea.
Former Manchester United manager Louis van Gaal froze out Valdes at Old Trafford
Coach Louis van Gaal claimed that the former World Cup winner had refused to play in a reserve team match, and his days were quickly numbered at Old Trafford after the arrival of Sergio Romero shortly afterwards.
And the now Middlesbrough keeper admitted that without the support of United Under-21 coach Alan Fettis, he could easily have given up the game for good.
Valdes now plays for Middlesbrough
"This was the biggest fight of my career and I won it", he said of his difficulties in recent times.
"The easy way out was to end my career but now I feel like a professional goalkeeper again.
"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."
Valdes joined Middlesbrough on a free transfer earlier this summer after his contract at Old Trafford expired.
The move sees Valdes link up with compatriot Aitor Karanka, who has also managed to attract fellow Spaniard Alvaro Negredo to the Riverside.
Middlesbrough's Alvaro Negredo celebrates scoring his first goal for the club
Negredo scored on his debut against Stoke City last weekend, as Boro earned a commendable 1-1 draw.
They take on North-East rivals Sunderland on Sunday, in their first league encounter since 2009.