Ronaldo started the game on the bench amid speculation surrounding his future at the club. According to Sky Italy, Ronaldo requested to not start the match and ESPN has reached out to Ronaldo's representatives for comment.
But after coming on, the 36-year-old forward thought he had headed his side to victory right at the death, only for VAR to rule Ronaldo to be offside.
A soft penalty conceded by goalkeeper Wojciech Szczesny six minutes after the break gifted Udinese a route back into the match before an even more calamitous error by the Pole let Gerard Deulofeu in to level in the 83rd minute.
Ronaldo had sat out of Juve's final preseason friendly in midweek, with the club saying the Portuguese was not fully fit.
He had been expected to start against Udinese but was left on the bench, with Juve vice-president Nedved saying before the game that fitness concerns had again kept him out and any talk that he had asked to miss the match to remain fit for a move away from the club was just speculation.
"Ronaldo is feeling well; I had talked to him before the game, telling him he would start on the bench," Allegri told DAZN. "He made himself available, he did well when he entered the pitch."
On the pitch, Juve did not need last season's top goalscorer in Serie A, as Dybala put a frustrating campaign last term behind him with an instinctive finish early on before Cuadrado finished brilliantly to make it 2-0 inside 23 minutes.
But the visitors then conceded a soft penalty, which was converted by former Juve forward Roberto Pereyra, as the hosts came back into the contest.
Szczesny then was far too casual on the ball, trying to flick it past Stefano Okaka, who made the block, and Deulofeu was on hand to sweep the ball into an empty net to stun the visitors.
The goal was initially ruled out for a foul on Szczesny, but VAR intervened to overturn the decision.
The video assistant referee's night was far from over. The cross from Chiesa, one of Italy's standout performers in their Euro 2020 success this summer, was perfect, while the leap from Ronaldo unstoppable.
The veteran looked to have snatched yet another victory for his side, but it was not meant to be as Juve, many experts' tip for the title this season, got off to a disappointing start.