The five-time Ballon d'Or winner was tipped to complete a move to Manchester City in the last knockings of the transfer window, only for United to swoop in and re-sign their former player.
Ronaldo spent three years at Juventus before sealing a return to Old Trafford 12 years after his initial departure to Real Madrid, but an agreed deadline to complete his exit from Turin was allegedly not met.
According to Cadena SER, Juve informed Ronaldo's agent Jorge Mendes that they were willing to sanction the 36-year-old's exit, but only if all the details were finalised by August 15.
The report adds that Juventus did not want less than two weeks to secure a suitable replacement for Ronaldo, but the Old Lady's 2-2 draw with Udinese on August 22 led the attacker to push for an exit and break the pact.
Ronaldo has already scored four goals in three games for Man United during his second spell at the club, including the equaliser in Sunday's 1-1 draw with West Ham United.