The Real Madrid and Portugal forward left Old Trafford in 2009 for a then world-record fee of £80million having spent six years with the Premier League champions.
Ronaldo - who has scored 31 goals in 21 games for club and country this season - was linked with a return to United in the close season, and Moyes has revealed he hopes to work with him in the future.
"Everyone at Manchester United would tell you he is the best in the business," he told MUTV.
"I have not been fortunate enough to work with him but there is still hope some day it might be possible."
In terms of current personnel, Moyes wants to get more out of midfielder Marouane Fellaini.
Despite a modest start to his United career after his switch from Everton, Moyes believes both he and Fellaini are on the same page when it comes to the Belgium international's form.
"He knows he can play better," the Scot said.
"(For) a lot of people who join Manchester United - it doesn't always happen as quickly as you would like. He needed to take time but we threw him in very quickly. Ideally we would have bedded him in more gently and at the right opportunities.
"But we had to put him into some big games at quite a difficult time for us."
While Fellaini's first two games for United both resulted in wins he then went four matches without victory - including defeats against Manchester City and West Brom.
Moyes feels Fellaini's introduction was something of a baptism of fire, but hopes he can grow to become a "bigger influence" on the team.
"When he came into the team we were not playing as well as we would have hoped," Moyes added.
"I think we are yet to see him properly. He can be a bigger influence. There have been a couple of games where I thought he threatened to be a big influence."
