Juan Mata’s public show of frustration after being substituted at Southampton on New Year’s Day was the beginning of the end for the midfielder at Chelsea.

The normally mild mannered Spaniard reacted angrily to Jose Mourinho’s decision to replace him with Brazilian Oscar shortly after half-time during the 3-0 win.

Mata ignored his manager as he left the field before taking his frustrations out on his seat in the dugout and throwing his arms into the air at St Mary’s.

His Chelsea team-mates calmed him down on the bench while Oscar, Mourinho’s favoured number 10 at Stamford Bridge, lit up the game with two assists and a goal.

The events of that day set the wheels in motion for Mata’s proposed move to Premier League champions Manchester United, which is expected to be concluded before the weekend.

For the past two seasons, Mata had been highly effective when deployed in the hole behind a central striker, so much so that he won back-to-back player of the year gongs.

But from day one he has never been the man for Mourinho.

Mata’s patient, calculated, midfield scheming doesn’t fit in with the brand of football the Portuguese manager is trying to establish at Chelsea.

That’s not to say Mata is a bad player. Far from it. Blues fans are still struggling to comprehend the expected loss of the World Cup winner, particularly to a rival club.

But he’s just not what Mourinho is looking for. That much has been evident from the moment Mourinho swept back through the doors in SW6.

Mata is at his best in the final third, that’s where he does his damage, but the returning Chelsea manager’s team requires players who track back.

Eden Hazard, Andre Schurlle, Willian and Oscar are all ahead of Mata in the pecking order and it’s no coincidence that they’re all more powerful players than the former Valencia star.

Mata played more than 63 games for the Blues last term but has been forced to sit out much of the current campaign, hindering his chances of making the Spain squad for the World Cup.

Like any player, the diminutive playmaker wants to be on the plane to Brazil this summer and that simply won’t happen unless Mourinho’s philosophy changes or Mata leaves this January.

The former will not happen and the wheels for the latter have already been set in motion. David Moyes is desperate for reinforcements to help secure a top-four spot for United.

Real Madrid are rumoured to be plotting a bid for Wayne Rooney, who was the subject of two bids from the Blues in the summer, and Mata’s arrival would help prepare for his departure.

Some will argue you should never sell to a rival, but there is a level of professional respect among the Stamford Bridge hierarchy for matter. They are receptive to his wishes.

Manchester United are now understood to have followed up their interest with a formal bid of £35million. One thing is clear,  there appears to be no going back.