United manager Mourinho has mostly relied on the services of Valencia at right-back since his arrival at Old Trafford but he is understood to be concerned over the club's long-term plans with the Ecuadorian approaching his 33rd birthday.
"It is an unbelievable opportunity for me", he said.
Michael Carrick and Manager Jose Mourinho of Manchester United celebrate after the EFL Cup Final match between Manchester United and Southampton at Wembley Stadium on February 26, 2017 in London, England.
He will now link up with Mourinho's coaching staff, but Carrick will not be a like-for-like replacement for assistant boss Rui Faria, who has chose to step down after 17 years working alongside Mourinho.
"The particular reason is that I think it will be Michael Carrick in the future, when he has his badges and his pro licence, when he makes the bridge from a player to an assistant", the Red Devils boss added.
The former England midfielder also said he meant to be a "pest" by picking the brains of as many people as possible as he embarks on his foray into coaching and potentially management in the future.
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United have beaten every single member of the top four at least once this season, but have also lost to all three promoted sides, as well as dropping points to two of the sides that have been relegated.
Neville went on to assess how United can close the gap on City next season, with Neville suggesting big changes are on the horizon as Mourinho looks to shake up his first team squad. "I'm sure that will be the case again".
"It's a gamble for him - he's saying he hasn't got anybody in that group already to replace Faria, whether he goes looking outside who knows? I'm well aware of that", he said.
The tweet read: "Carrick, wonderful person and sensational player".
"I had the issue when I signed and it was all about Roy Keane, but I was never going to replace Roy Keane because it's not how I am". There will be other players.
"You've got to go step by step", he said.