The pessimistic view when Ronald Koeman moved to Southampton in 2014 was that he had accepted a hospital pass, bearing in mind they were struggling to absorb an intense and bruising talent drain on and off the pitch. In reality it was more like a ball from Xavi.

It is understandable that the mood at Everton is celebratory after days of protracted negotiations ended with them securing Koeman as Roberto Martínez’s replacement. After Martínez allowed his commitment to possession football to spill over into a dogmatism, a blind spot towards defending that eventually proved to be the Spaniard’s undoing, now they have a manager who has demonstrated an ability to strike the right balance between defence and attack during his impressive spell at Southampton.

The way that Koeman has exceeded expectations in the past two seasons has elevated his reputation and explains why Everton went to such lengths to woo him. Southampton’s late surge to finish sixth, featuring wins over Liverpool, Manchester City and Tottenham Hotspur during the run-in, means they will go straight into the Europa League group stage next season. A Boxing Day thrashing of Arsenal emphasised the attacking threat offered by Koeman’s side, while consecutive 1-0 wins over Manchester United at Old Trafford demonstrated their adaptability and counterattacking qualities.

Looking on from a distance, Southampton’s position was perilous when Koeman arrived. Losing Mauricio Pochettino to Tottenham was exacerbated by the sales of Calum Chambers, Adam Lallana, Rickie Lambert, Dejan Lovren and Luke Shaw for a combined £88.5m, and the more doom-laden predictions had Southampton earmarked for a return to the Championship. Instead they finished seventh.

Yet a striker cannot finish off a move if the final pass is not executed properly. If Lionel Messi’s ruthlessness in front of goal made the assists from Xavi and Andrés Iniesta better, their creativity also lifted him to new heights. In the same way, while Koeman has been an undoubted success since replacing Pochettino, he could not have thrived without the foundations put in place by Southampton. They were able to present him with a thriving youth system, a new state-of-the-art training ground, boardroom nous, a strong recruitment team and the basis of a good squad. Southampton have been here before and recent history has shown that they have reacted to setbacks by emerging stronger.

In that context it is natural to wonder whether Koeman has made the right choice to leave them for Everton. That he has is not a huge surprise – he has never given the impression that this was a long-term project for him and has made no secret of his desire to manage Barcelona one day – but that has not stopped doubts being raised about whether this is a sideways move.

The debate is likely to inflame supporters of both clubs and Southampton’s can be forgiven for questioning why Koeman has left them for a team who finished five places and 16 points below them last season. On those numbers alone, it represents a downgrade. Several of Southampton’s most important players have just signed new long-term contracts and there is European football to look forward to.

Ronald Koeman is leaving Southampton for a team who finished five places and 16 points below them last season. Photograph: Glyn Kirk/AFP/Getty Images

At the same time, however, Koeman had one year left on his deal and Southampton’s chairman, Ralph Krueger, did not sound overly optimistic that talks over an extension would end well when he met the media in April. There is an image problem. Southampton have developed a reputation as a selling club – Sadio Mané is a £30m target for Liverpool – and it is fair for Koeman to wonder whether he can take them any further.

Everton are a bigger club. In light of Leicester City’s Premier League league title win, it would be wrong to say that Southampton are at the peak of their powers, but the potential at Goodison Park is vast. Everton have won nine league titles, five FA Cups and a European Cup Winners’ Cup, compared to a solitary FA Cup for Southampton in 1976. Everton’s history is alluring, the fanbase is large and the thirst for success is huge.

What swung it for Koeman was the promise from Everton’s new majority shareholder, Farhad Moshiri, that he will be given the biggest transfer budget in the club’s history, plus the proceeds from the anticipated sale of Romelu Lukaku and possible sale of John Stones. Although reinforcements are needed in various areas, the presence of Ross Barkley, James McCarthy, Gerard Deulofeu and Seamus Coleman gives Koeman a solid base.

But the grass is not always greener. Koeman should be careful. A big budget offers new possibilities and it also brings pressure. Puncturing the elite will not be easy and if Everton invest heavily, patience may be in short supply if Koeman does not hit the ground running. Southampton could point out that he was not looking so clever during a winter run of eight defeats in 10 matches last season. How would a similar slump be viewed at Everton?

The chance to spend offers a greater chance of success and Everton have flexed their financial muscles. Yet it should not be forgotten how far Southampton have progressed thanks to a vision centred on careful planning and gradual, measured improvement, or that they seemed to cope just fine the last time a club from Merseyside came calling for players. It is a good place for a manager to develop. One day they may find someone loyal enough to see it through to the end.