United triggered a one-year contract extension for the 31-year-old Valencia this week, tying him to the club until 2018.
Initially signed as a winger in 2009, Valencia has established himself as a key cog in Mourinho’s defence, making 23 starts across all competitions this season.
And Mourinho is overjoyed with his performances, which he believes have put him at the top of his new position and made him fully deserving of his new deal.
“I don’t think of it is a reward, I don’t see it in this perspective,” Mourinho said to MUTV.
“I just think he is the best right-back you can have. There is no better right-back in football. It is just for us to keep the best. I don’t think it is a reward.
“It is a privilege for us to have such a good player and such a good man.
“Antonio is playing better than ever, I think. This season he is playing magnificently.”
Mourinho, who revealed last year he had once wanted to sign the Ecuador international for Real Madrid, saw Valencia’s potential to star as a right-back at an early stage.
“He was not playing at right-back and yes, I wanted him to play at right-back,” the manager said of his failed bid to take him to Spain.
“But Manchester United was intelligent and strong. They immediately stopped any feeling about any possibility.”
Valencia’s major honours at Old Trafford include two Premier League titles, in 2011 and 2013, as well as FA Cup and EFL Cup medals.