Speaking after City's penalty shoot-out victory over Liverpool in the Community Shield, Guardiola congratulated United on the capture of Maguire, who arrived at United's Carrington training complex to complete his transfer from Leicester City on Sunday.
The champions signed Rodri, Angelino and Zack Steffen for a combined £80.8million during the close season, and United are expected to pay £80m for England centre-back Maguire.
Backing City's local rivals to challenge for the Premier League this season with Maguire in their defence, Guardiola told reporters: "Maguire is an excellent, top-class player.
"We were interested but we could not afford it. United could afford it.
"He had an incredible World Cup, he's strong in the air, he's good with the ball in the build-up, he can drive the ball, he's fast, so fast. Congratulations to United.
"At the beginning of the season I think there are five, six contenders [for the title].
"It doesn't matter what happened in the last four, five years [with United]. Always, at the beginning [of the season] they are contenders."
Guardiola gave a positive assessment of an injury to winger Leroy Sane, who lasted just 13 minutes of the Community Shield before suffering an injury.
The Germany international has been linked with a transfer to Bayern Munich in the close season but Guardiola has expressed his desire to keep Sane at the Etihad Stadium.
Asked about Sane's condition, Guardiola replied: "The first impression was not good but I think it's not bad.
"Honestly I don't know, we will wait to see what the doctor says but I think he will be okay."