For many Madrid fans, the striker's Italian getaway - during rest days granted by Real Madrid to injured players, including Mbappe, Thibaut Courtois and Arda Guler - was the final straw.
United behind a shared image showing Mbappe with a "Fuera (out)" stamp over his face - originally posted by a satirical account and reposted in the comments section of Real Madrid's official posts - some fans are openly calling for the French superstar to leave.
An online petition has also been circulating on social networks, urging Madrid fans to "make their voices heard". "If you believe change is necessary, do not remain silent: sign this petition and defend what you believe is best for the future of the club," it reads.
Fuel was added to the fire by widely shared photos of Mbappe alongside actress Ester Exposito, posted on social media as Real Madrid were preparing for a crucial La Liga match against Espanyol last weekend in an effort to delay Barcelona's title celebrations.
Real Madrid won that match 2–0, and coach Alvaro Arbeloa later insisted that "each player does what they consider appropriate in their free time" and that it was "none of my business". However, several of his comments were perceived as indirect criticism of his leading scorer.
Sweat and mud, not tuxedos
"We didn't build Real Madrid with players who play in tuxedos, but with players who finish matches with shirts full of sweat and mud, through effort and sacrifice," he said.
Madrid is "a club where, fortunately, no player has ever been, is, or ever will be bigger than Real Madrid," added Arbeloa.
Contacted by AFP, Mbappe's entourage said "part of the criticism is based on an over-interpretation of elements linked to a recovery period that is strictly supervised by the club", and bears no relation to "the reality of the commitment and daily work Kylian puts in for the team".
The France captain, accustomed to sometimes excessive scrutiny, addressed his mindset earlier this season on The Bridge, a podcast hosted by his Real Madrid and France teammate Aurelien Tchouameni.
"I've reached a point where you're going to get criticised whatever you do, so you might as well do what you want - at least then you stay true to yourself," Mbappe said.
Isolated in dressing room?
With 41 goals in 41 matches in all competitions this season, Mbappe remains by far Real Madrid's leading scorer. He has nonetheless come under heavy criticism since returning from injury in mid-March, with some observers accusing him of an overly individualistic approach.
According to Spanish media, Mbappe - frustrated by a second season at Madrid without a major trophy - is becoming increasingly isolated in the dressing room, despite having established himself as a leader earlier in the campaign.
Some fans and pundits have pointed to post-match comments from fellow stars Vinicius Junior and Jude Bellingham after Sunday's win as signs of a growing rift with the France captain.
"We fought and we played together. When we play like that, we are better and stronger," Vinicius told Real Madrid TV, while Bellingham urged his teammates on Instagram to stay "together" and fully committed "until the very last whistle" of the season.
Friendly exchanges between Vinicius and Bellingham on social media - the pair won the Champions League together before Mbappe's arrival - have further fuelled speculation of a new clash of egos within the Madrid squad.