The Citizens were linked with Maguire last summer and until recently, but Guardiola suggested to reporters that United will be contenders for the Premier League crown after they complete the transfer:
"We were interested in him, but we could not afford it and United could afford it.
"Maguire is an excellent, top-class player, a top-class central defender. He has all the qualities to work for United. They are contenders, with the players they bought like Maguire.
"I don't care if people say we are going to win titles, and I don't know if it'll be like last season with two real contenders."
Guardiola was speaking at a press conference after City defeated Liverpool 5-4 on penalties following a 1-1 draw in the Community Shield on Sunday.
The £80 million figure will make Maguire the most expensive defender in football, surpassing the £75 million Liverpool paid to sign Virgil van Dijk from Southampton in January 2018.
BBC Sport's Simon Stone provided context behind United's move:
Simon Stone @sistoney67
On Maguire. Fee high but OGS felt combination of ability on ball + leadership was what he needed. As recently as three weeks ago, Man Utd had other options but reached point where Maguire was only one + by middle of this week were confident. Leicester will presumably go for Dunk.
The defending Premier League champions could soon complete their second major signing of the summer after football journalist Fabrizio Romano reported progress in their pursuit of Juventus full-back Joao Cancelo:
Football writer Andy Mitten tweeted Maguire's desire to move to Old Trafford was key to the transfer:
The Red Devils finished sixth in the Premier League last season and relied on inconsistent central defenders including Victor Lindelof and Eric Bailly, as well as England internationals Chris Smalling and Phil Jones. Marcos Rojo is another option for Solskjaer, but he was sidelined for most of the 2018-19 campaign.
Ex-City stalwart Vincent Kompany left the club this summer to join Belgian outfit Anderlecht as player-manager. However, Guardiola still has a stable of talented centre-backs in Aymeric Laporte, John Stones and Nicolas Otamendi, while Cancelo's arrival could signal a move into the middle for full-back Kyle Walker.
United will look to complete Maguire's signing as soon as possible in the hope he'll be ready to make his club debut in their Premier League opener at home to Chelsea on Sunday.