It's a fact that 'free' transfers in football are, all things considered, quite expensive. Wages, signing fees, agent demands and general costs make any such deal have a sizeable imprint on a club's balance sheet.

Still, Paris Saint-Germain's signing of Gianluigi Donnarumma for a big fat zero in transfer fees sounds like a sensational piece of business – particularly after his Euro 2020 heroics.

The Italy goalkeeper has left Milan, his boyhood club, to join Mauricio Pochettino's side where he hopes to be able to challenge for the biggest club trophies, most obviously the Champions League.

Certainly, Donnarumma is right to aspire to the top prizes. This is a player who made 200 appearances for the Rossoneri by the age of just 21 years and 146 days, who was valued at €170million by agent Mino Raiola when he was just 16.

PSG have secured a bargain, for sure – but is it one they really needed? Incumbent number one Keylor Navas certainly did not seem to think so when he posted to Instagram "gift your absence to those who do not value your presence" shortly after the Donnarumma rumours first emerged.

As Opta data shows, Navas, a four-time Champions League winner, may well have a point.

First, it's important to stress once again that Donnarumma is an excellent signing. PSG are recruiting a highly experienced yet young goalkeeper without paying a transfer fee; a goalkeeper whose market value is only likely to increase over the coming decade – indeed, it has in the past week alone – should Les Parisiens decide to cash in at any point.

However, it's also true that, if the short-term goal is simply to improve the first XI with a view to winning the Champions League, replacing Navas with Donnarumma may not be a fool-proof move.

In 2020-21, Donnarumma kept 14 clean sheets in 37 matches in all competitions. Navas also managed 14 shut-outs, having played in eight fewer games. The Costa Rican conceded only 18 goals in those matches, whereas Donnarumma let in 38.

Donnarumma produced more saves (92) than Navas (74), but the older man's save percentage was far higher: 80.43 per cent, compared to 70.08. In fact, Navas boasted the highest save percentage of any keeper in Europe's top-five leagues last term who started at least 10 matches, a fraction above Atletico Madrid's Jan Oblak (80 per cent). Donnarumma came 25th in those rankings.

Navas' distribution was better, too: his passing accuracy (85.65) was better than that of Donnarumma (77.04), with only three regular keepers in Europe's top-five leagues posting better numbers. Donnarumma did at least surpass Navas for keeper sweepings, which are defined as any time a keeper anticipates danger and rushes off their line to try either to cut out an attacking pass (in a race with the opposition player) or to close down an opposition player: he completed nine of nine such 'sweepings', with Navas on five out of five.

Going further and looking at expected goals on target – a way of building on expected goals that takes into account the quality of the attempt on goal – we can highlight how well these keepers have done to keep high-quality shots on target from going in. This is done by subtracting goals conceded (excluding own goals) from xGOT conceded, giving us 'goals prevented'. And it's another big tick for Navas.

In Ligue 1 last term, if we exclude penalties and own goals, Navas let in 17 goals from an xGOT conceded total of 24.1, giving him a figure of 7.1 goals prevented. In other words, he would reasonably have been expected to concede roughly seven more goals based on the quality of shots on target he faced. Meanwhile, Donnarumma conceded 35 from an xGOT of 34.1, giving him a goals prevented figure of -0.9 – not terrible, but nothing to shout about.

Last season's figures were not isolated, either. If you look back over Donnarumma's 215 club appearances since his Milan debut on October 25, 2015, Navas' consistency as a shot-stopper is superior.

Donnarumma conceded 227 goals for Milan (excluding own goals) in all competitions from an xGOT figure of 244.4, giving him a goals prevented tally of 17.4. That's a strong return – better, for instance, than Manchester United's David de Gea (16.6) and Liverpool's Alisson (16.1) – but, again, it's lower than that of Navas (20.3).

For balance, both of them are a long way behind the top performer in this metric over that time frame. That man, unsurprisingly, is Jan Oblak of Atletico Madrid, with a quite remarkable goals prevented figure of 44.5.

It's as yet unclear what Navas' plans will be now that Donnarumma has moved to Parc des Princes, but a keeper with his pedigree and medal collection is unlikely to settle for a back-up role. And nor should he: Donnarumma might be PSG's future, but Navas does not deserve to be dispensed with in the present.