If Jose Mourinho starts next season as Paris Saint-Germain coach, Manchester United executive vice-chairman Ed Woodward need look no further than this summer's failure to sign Ivan Perisic from Inter Milan as the moment that his manager's dissatisfaction was ignited at Old Trafford.
Sources have told ESPN FC that United should be taking the speculation surrounding Mourinho and PSG seriously and that the French club will look to take advantage of any sign of disharmony between the former Chelsea manager and the Old Trafford hierarchy.
PSG already have a head coach in Unai Emery, of course, and the Spaniard has the team sitting on top of Ligue 1 and already assured of a place in the Champions League knockout stages. But the former Sevilla coach is also battling to manage a squad crammed with big stars, and even bigger egos, and sources have told ESPN FC that Emery, having failed to win the title last season, will be vulnerable if he does not win the club's first Champions League trophy in May.
Mourinho, with two Champions League titles on his CV at FC Porto and Inter Milan, is highly-regarded by PSG's Qatari owners and did little to subdue their interest by admitting publicly last month that he would not end his career at United, before instantly speaking of the "magic" and "special" atmosphere of Paris.
With Portugal, Italy, Spain and England under his belt, Mourinho dreams of conquering another league and he has yet to manage in France, but with Pep Guardiola threatening to overshadow him in Manchester for the foreseeable future, a move to PSG would enable him to take on his old rival in Europe with the same limitless budget that Guardiola enjoys at Manchester City.
And despite Mourinho's big-spending at United since succeeding Louis van Gaal in the summer of 2016 -- he has spent more than £300 million on new players in less than 18 months -- the failure to go further and meet the £50m asking price for Perisic, at a time when City were splashing out more than £50m each on full-backs Kyle Walker and Benjamin Mendy, has become a long-standing source of irritation for the United manager.
Sources said that Mourinho was exasperated by the delay in signing Nemanja Matic from Chelsea, with last-minute haggling over a small difference in valuation denying the player the chance to participate in the summer tour of the United States. Matic eventually arrived in August.
Perisic, meanwhile, remained at Inter, despite Mourinho making it clear to Woodward at the end of last season that the winger was one of the four targets required to boost the squad.
The Croatian was keen to make the move, but at 28, there were concerns at United over the size of the fee for a player with little resale value. A compromise could not be reached with Inter, whose valuation was, according to sources, no greater than £5m more than United's.
ESPN FC reported in August that Mourinho had chosen against selling a member of his squad to fund a move for a fourth summer signing (Perisic) in order not to upset the harmony of the group, preferring to wait until January.
But the requirement to sell before he can buy remains and Mourinho is once again reluctant to do so. Perisic, meanwhile, now appears out of reach following his decision to sign a new contract with Inter.
Without the width and balance that Perisic would have provided, however, United have lacked variety in games against more organised opponents and the recent decline in goals -- £75m signing Romelu Lukaku has now failed to score in seven games -- has coincided with City opening up an ominous eight-point lead at the top of the Premier League.
That situation has darkened Mourinho's mood, as has Marouane Fellaini's ongoing wait for a new contract to replace the final six months of his current deal.
Sources told ESPN FC in April that Mourinho was set to recommend to the board that Fellaini be handed a new deal, but the Belgian's future remains unresolved.
The air of discontent around Mourinho has been noticed within the club, however, and sources have spoken of not knowing which Mourinho will turn up at the Carrington training ground on a daily basis, whether it be the upbeat version or the moody one, prepared only to engage with his small group of assistants.
There is a view that Mourinho's demeanour is merely a reflection of his frustration at recent performances and results, with those prepared to observe the bigger picture pointing to United sitting second in the table and top of their Champions League group with a 100 percent record.
But the manager's outspoken comments about the club's supporters and thinly-veiled complaints about summer transfer dealings have not projected an image of a manager happy with his lot.
And with PSG seemingly on a mission to land blows on all of their major European rivals -- they beat Real Madrid to Kylian Mbappe, took Neymar from Barcelona and thwarted City's move for Dani Alves -- the French club will know that landing Mourinho from United would be another huge statement.
The question now is whether United believe Mourinho is worth fighting to keep or whether, ultimately, he is becoming more trouble than he is worth.