The Chelsea manager does not usually sell a key player to a rival but Roman Abramovich has allowed the Czech goalkeeper to get the move he wanted.

José Mourinho finally laid bare his feelings at the beginning of May. Perhaps it had been all the sentimental talk that made him snap. To the Chelsea manager there can be no room for that sort of thing in the cut-throat world he inhabits. Or, maybe, it had been the realisation that Arsenal might just be a serious contender for the Premier League title in 2015-16.

Mourinho did not mince his words. If the decision were down to him, Petr Cech would stay at Chelsea for the final year of his contract and, yes, he would be made to do so against his will, if necessary. Moreover, and as importantly, if the decision were down to Mourinho, the goalkeeper would not be going to Arsenal.

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“The club is more important than the player and, as was true this season, the club needs two very good goalkeepers,” Mourinho said. “For me, decision No1, Petr will stay. After that, second point, to go but not in England. The option I don’t want is for him to go to an English club.”

It was 8 May and Arsenal were second in the table to Chelsea – albeit 13 points behind, with a game in hand. Chelsea had made sure of the title but Arsenal had won 18 of 21 fixtures in all competitions. It was becoming clear at the time – and the impression would subsequently harden – that Arsène Wenger’s principal transfer targets in the summer would be a goalkeeper and a defensive midfielder. In other words the Arsenal manager considered an upgrade between the sticks – which he got on Monday when Cech signed his four-year deal at the club – to be one of the missing pieces in a potential championship challenge.

To Mourinho the thought of granting the piece to Arsenal was faintly ludicrous. And so he made his feelings plain. At that press conference back in May he put the ball firmly into Roman Abramovich’s court. The only thing to conclude now, as Arsenal celebrate the capture of Cech, is that the Chelsea owner promptly fizzed a passing shot beyond him.

There is discomfort on plenty of levels at Chelsea over the sale of Cech to a direct rival – and not just from Mourinho, whose frustration and point-making was reflected in his demand for an Arsenal player to be included in part-exchange; he wanted Theo Walcott or Alex Oxlade-Chamberlain, which was never going to happen. There is the acknowledgment among Chelsea directors that the transfer is far from ideal and many of the club’s supporters are gutted.

The mind goes back to the deadline day in September 2013, when Arsenal thought that they had a deal in place to take the striker Demba Ba on loan from Stamford Bridge. Chelsea strung them along before they pulled the plug at the last moment, leaving Arsenal with no time to pursue an alternative. “Why would we want to help a rival?” a club source said.

This is the kind of intransigence that might be expected of the elite. When Chelsea have sold a big player to a Premier League club in the recent past – think Daniel Sturridge to Liverpool in 2013 and Juan Mata to Manchester United last year – it was because they did not consider them to be rivals. Mata did not fit into Mourinho’s system. Had the midfielder done so, Chelsea would not have sanctioned the move.

Cech’s case is different. Mourinho loves him and had said on several occasions last season that he felt the club had – in Thibaut Courtois and Cech – the best two goalkeepers in the division. “Without Petr we would not be champions,” Mourinho said. “Without the points in the games he played, we would not be champions.” Cech made seven appearances in the league and he conceded only twice. Chelsea won all seven.

Mourinho is aware not only of the need for high-quality back-up but of the difficulty in replacing Cech with somebody who is even comparable. He likes Asmir Begovic of Stoke City but it is a tough sell persuading any top goalkeeper to sign up as the No2 to Courtois.

Abramovich’s hitherto unseen benevolent side has allowed Cech to get the move he wanted, which keeps the 33-year-old and his young family in London. Abramovich felt he owed it to Cech to let him choose his next club, on the basis of his 11 years of outstanding service. Cech’s professionalism has always been total, particularly last season when he found himself in the unfamiliar role of understudy. But Abramovich has been more benevolent than Mourinho would have liked and Chelsea must hope that the decision does not return to haunt them.

Cech is almost 18 months younger than Edwin van der Sar was when he moved from Fulham to Manchester United in 2005 – and look at how that ended up. Van der Sar won virtually everything at Old Trafford over six seasons, including the Champions League in 2008, when he helped United to beat Chelsea on penalties in the final. Goalkeepers tend to age much better than outfield players and Cech, who has looked after himself with diligence, believes he has at least four more seasons at the highest level.

Has Cech still got it? Wenger thinks so. Cech’s form remained impressive last season and this was when he was in and out of the Chelsea team, to get into a groove. He made nine appearances in the various cup competitions, conceding nine goals – four of which were in the FA Cup defeat against Bradford City. Chelsea do not give up many chances but, when called upon, Cech was polished. He had the highest save percentage in the Premier League, according to Opta, of the goalkeepers who played in more than one game.

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Arsenal are getting a winner and a leader, an overall improvement on David Ospina and Wojciech Szczesny, and they might also enjoy the sensation of nabbing a player from a rival, which has happened to them in recent years. In some respects the boot that kicked them with Robin van Persie and Samir Nasri is on the other foot.

There are people close to Abramovich who say that, when he buys something, he buys it to keep. It is part of his makeup. Since his takeover at Chelsea in 2003 he has rarely sold a highly rated player. But that is changing. In the era of financial fair play, Chelsea are learning when to sell. They believe the sale of David Luiz to Paris Saint-Germain was excellent business and they have been happy with the outgoing deals for Mata, Romelu Lukaku, Kevin De Bruyne and Ryan Bertrand.

Cech becomes only the sixth Chelsea player to move directly to Arsenal, following Bill Dickson (1953), George Graham (1966), William Gallas (2006), Lassana Diarra (2007) and Yossi Benayoun (loan, 2011). The risk on this one looks pronounced.