Opinion has been divided on the wisdom of Chelsea selling Nemanja Matic to Manchester United for £40m. There are those who say top teams, especially defending champions, should never sell to direct rivals, and those who believe the money was too good to refuse down for a player who has just turned 29.

It is being argued that Chelsea have just signed an upgrade in Tiémoué Bakayoko for the same price, at least a player Antonio Conte regarded as an upgrade in the same position, and at just 22 the promising France midfielder offers many more years of service than Matic, whose sale effectively means the former Monaco player has joined for free.

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All of that makes sense, yet what is equally undeniable is that United were delighted to land Matic, a player José Mourinho knows well, and that along with Romelu Lukaku the two new signings made highly promising debuts in the first game of the season at Old Trafford. Never mind for the moment that United were only playing West Ham, or that it is usually unwise to read too much into the first game of the season, the question to be asked is why Chelsea are making life so easy for their former manager and his new team.

Lukaku, let us not forget, was originally a Chelsea player and was widely expected to return to Stamford Bridge when he left Everton. Perhaps so widely expected that Chelsea may have let the grass grow under their feet and failed to anticipate United making a pre-emptive strike. Conte was understood to be furious – when is he ever otherwise? – and has appeared to be unhappy with his employers ever since and ended up buying Álvaro Morata, all too clearly a second-choice target and one who was not trusted to start on the opening day.

We all know how that went. Morata did come on to start a belated fightback as Chelsea lost their bearings and most of their composure against Burnley, but as striking debuts go it was nothing to compare with Lukaku – “the manager wants me to be a leader for the team” – and the beginning-of-a-new-era feeling most observers noted at Old Trafford.

Matic had as much to do with that as anyone, and it is probably worth bearing in mind that Mourinho was being linked with Eric Dier for most of the summer because he originally imagined the Serb would be off limits. “Generally I do not like to chase impossible targets,” the United manager explained ahead of the West Ham game. “There is no point being seen to make moves for players you cannot get. I could see when I first came to United that we needed a Matic-type player, but there are not many around and I did not think Matic would move. Then all that changed with a phone call. I heard that he wanted to join United and could make it happen.”

So either Chelsea set the ball rolling or allowed Matic to do it himself. Mourinho did not need asking twice. Like Lukaku, Matic had once been a Chelsea player who had struggled to get a first-team look-in. He was sold to Benfica in 2011 as part of the deal that brought David Luiz to London, and it is a measure of how highly he was rated by Mourinho that when the manager returned for a second spell at Stamford Bridge one of the first things he did was buy him back.

So Chelsea could not have been surprised by Mourinho’s continuing admiration for the midfielder. Perhaps not many other managers would have sanctioned £40m for a player of his age, but in the general scheme of things that money does not represent a stumbling block for United and even at that price Mourinho’s eyes must have lit up like a child presented with a birthday treat.

“He is one of those signings you can make where there are no question marks,” Mourinho said. “He’s not an inexperienced young guy coming to a new country. He’s a very consistent player with five years in the Premier League behind him. You can trust him to perform.”

The noises coming out of Chelsea at the moment suggest someone at the club thought differently. Should Matic perform through the season as impressively as he started on Sunday there will be a certain amount of embarrassment at Stamford Bridge and a few questions to be answered.

Most of the faults now being listed as reasons why his sale to United made good sense – it has been hinted that he is insufficiently mobile, ponderous on the ball and tactically inflexible – will strike Mourinho as positive recommendations. He knew precisely what he was getting, a disciplined performer who plays an uncomplicated yet effective game, one who has won two Premier League titles in the past three years and probably still has enough in the tank for more. As an added bonus, Matic is a muscular 6ft 3in, perfect for the physically imposing side Mourinho seems to be putting together at Old Trafford.

Chelsea still have N’Golo Kanté, and Bakayoko when he regains fitness, yet their hierarchy still seem to be struggling to come to terms with giving Mourinho exactly what he wanted. While every club has a duty to move out players to make way for younger or classier replacements, it is not yet clear that is what has happened at Chelsea.

Matic would seem to most eyes to have plenty of shelf life left, and the possibility remains that Chelsea have not only strengthened a direct rival but weakened themselves.

They deserve to have all their best players fit and available before being properly judged, but they made a sloppy, inauspicious start and now have suspensions as well as injuries to deal with. Some are blaming Conte for a lack of leadership, others suggest Roman Abramovich could be more proactive, and the absence of clarity over who is making the decisions and why does not augur well for a successful title defence.

Conte’s great achievement last season was to simplify things. He took over a team with problems and found a solution everyone understood. Now it seems the problems are back and the future foggier than ever. Conte has even admitted as much. Over at United, on the other hand, the clouds are lifting. The consensus is that Mourinho now has a team capable of playing in the way he wants to play. A big, powerful yet graceful one. Chelsea need to keep an eye on that, because to a large extent they have been facilitators.