Comment: Why are Chelsea willing to sell Juan Mata to Manchester United?

It was in Jakarta, on the last leg of Chelsea's tour of south-east Asia in July, that the conversation turned to Juan Mata and, more specifically, what it was that Jose Mourinho expected of the man who had been the outstanding player under Rafa Benitez.

First Mourinho mentioned the fact, often repeated since, that Mata had won both player of the year trophies the previous season – one voted for by the fans, the other by the players. In fact, Mata has won the players' award for the last two years and this seemed to have made an impression on Mourinho, who acknowledged the status of the Spaniard.

"That means a lot," Mourinho said at the time. "People love him, but also the fellow players love the work he did for the team. Of course, he fits in my plans. I know him quite well. I have my idea about him. I have my idea about where and how he produces better, and where he has more difficulty. But we are here to try and keep him performing when he is, and try and help him to perform better in the situations where he finds it a bit more difficult."

Even at the time it was a remark that grabbed the attention. There had been rumours Mata was not the kind of player Mourinho would ordinarily have signed and here was the new Chelsea manager implying exactly that. Mata, who had won the Champions League and the Europa League in his two seasons at Chelsea, was important. But he could be improved.

Looking back six months on, as the Chelsea hierarchy determine their likely response in the event of a £40m-plus bid for Mata from Manchester United, it is clear that there were doubts from the start. Just how quickly Mata has fallen from grace at Chelsea has surprised even those at the club, although they agree that his current role as non-playing substitute is untenable in the long-term.

It is for that reason they are considering what to do in the event of a United offer. From the perspective of the demands of Uefa Financial Fair Play, and the prospect of obtaining premium value, selling Mata makes sense. But the club also recognise that there are few assets like the 25-year-old and it only takes one injury for him to be first choice again.

Chelsea acknowledge that difficult choices have to be made, that not every player is suited to a life on the bench to be called upon when needed, that sometimes a bold decision can have an energising effect on a squad.

At Chelsea the feeling is that Mata's late arrival to join up with the squad in pre-season, having played at the Confederations Cup, cost him dear. He was not the only one who joined up after the Asia tour for the games in America in early August. So too did Oscar, the man who has taken his place, as well as the likes of Fernando Torres, David Luiz and Cesar Azpilicueta.

But with Mourinho already doubtful about Mata it meant that at a crucial time, when the new manager was putting together his team, imposing a style and expectations, Mata was absent. He started in the second game of the season against Aston Villa but did not play a full 90 minutes until the Capital One Cup third-round tie against Swindon on 24 September. Since then he has played a full 90 minutes just twice.

Inside the club, the indication is that Mourinho told Mata over the opening months of the season that he should be patient and his run in the team would come. But that has not happened and both parties are aware that to continue saying the same would be to insult the player's intelligence.

If Mata is to leave, the club and Mourinho want it to be on the best of terms. There remains a great deal of respect for a superb professional who has more than delivered on the £23m fee paid to Valencia for him in August 2011. Chelsea had been offered Mata the previous two summers but only turned their attention to him when their efforts to sign Luka Modric and Neymar came to nought.

As with all major decisions that encompass so many factors there is a possibility that it could cost Chelsea in the future. Nevertheless, the consensus is that the current situation, with such a popular individual on the bench, only creates pressure for both manager and player. Mata's angry reaction to being substituted on New Year's Day raised the tension.

Why does Mourinho not play him? He has been frank about the responsibilities of the attacking three behind the striker to apply pressure. As he said after Sunday's victory over Manchester United, his is a team "that runs a lot". "We press very high on the pitch. My attacking players have big distances to run between the initial pressure and the way they compact with the defensive players." That is not Mata's natural game.

Mourinho has eulogised the work rate of Oscar who, if the Chelsea manager was asked to name his most influential attacking player now, would be the only individual with a chance of pipping Eden Hazard. Oscar and Hazard have been the most willing participants in the Mourinho revolution.

There is a possibility, those at the club acknowledge, that Mourinho's system could one day be refined to include Mata. But given the various pressures upon them, of FFP and the tension of an unhappy player, waiting for things to change is not an option. They have to make a decision on Mata's future now and, with the potential to earn their biggest transfer fee, there is a good chance he will leave.