They say form goes out the window for a derby, but is that cliché true?

Most cliches are based on a truism but that trope never sits right with me. In my youth, during the 1980s, when arguably Scottish football peaked, derbies were hard fought and difficult to call as there wasn’t a gulf between Celtic and Rangers or indeed Aberdeen, Dundee United and occasionally Heart of Midlothian.

In the 1990s, as Celtic were riven with poor ownership and then sustainable renewal under Fergus McCann, and Rangers were betting the farm on glory, the light blues generally prevailed despite the odd Celtic upset. During the early part of the 21st Century, while Celtic backed Martin O’Neill before stepping back from an unequal arms race, it was truly a 50-50 call most years. This Sky Television era may be where the trope originated.

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Since 2012, Celtic’s rivals have struggled to get to where David King wanted to – namely Rangers winning 55 per cent of the time - apart from the unprecedented COVID-19-impacted season. During Rangers' solitary title season, they bossed a chaotic Celtic side with four wins out of five in all competitions, a run that the Europa League finalists preceded with three wins out of the previous six. Post Ange Postecoglou’s initial injury-hit opening loss at Ibrox, the only blue success was in a dead rubber at the end of last season where the much-changed champions had one eye on an impending cup final.

Since 2015 there have been, arguably, only three upsets. Two were Scottish Cup semi-finals. The 2016 edition was won by the Govan side on penalties. A side containing Andy Halliday, Gedeon Zelalem (nope, me neither) and Lee Wallace caused such mayhem amongst the Rangers directors and corporate guests that Dermot Desmond was moved to recruit Brendan Rodgers for the next campaign. The 2022 semi-final settled by Carl Starfelt’s injury-time own goal was also out of keeping with the time. On either side of this triumph, Rangers failed to win 12 derbies. And finally, Celtic’s 1-0 win at Ibrox this season with a Kyogo Furuhashi goal was a minor upset given Celtic’s incoherent injury-hit start to the season up against the hope and expectation of Michael Beale’s “revolution”.

That’s perhaps three “shocks” out of 40 in the new derby era. Celtic, champions in 12 of the last 13 seasons, have won 23 and drawn seven of those 40 ties. These games tend to go to form.

Trending xG

Using the staple of expected goals, StatsBomb plots the trends for and against over the current league season for both clubs.

This shows the extent to which Brendan Rodgers is improving his Celtic side over time. The trends are based on a ten-match rolling average xG for and against.

At the start of the season, the Celtic differential was a rather unhealthy 1.0 xG. Why unhealthy? With an xG differential of one or less, there is scope for variability such as good/poor goalkeeping, refereeing mistakes, good/bad finishing to have an undue influence on outcomes. For example, during the dominant 2022-23 season under Postecoglou, Celtic set out with an xG differential of +2.0 xG. This allows the team to absorb much negative variance. It essentially de-risks games to a significant extent.

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As the season has climaxed in another title success, Celtic’s ending xG differential is just under 1.5, healthy enough. But the trends are all in the right direction. Under Postecoglou, the trends were both in the wrong direction and Rodgers has had to deal with significant talent outflow and early season injury woe on top of implementing his style of play.

As the season finishes, he has made the best of what he has in that regard and the team won all five post-split fixtures and scored 27 goals in the last nine league matches. Rangers changed manager partway through the season as Philippe Clement replaced Michael Beale. Here are their trends.

When Beale was removed the rolling xG differential was just over 1.0 xG in the Blues’ favour. At season end it is exactly 1.0 xG differential. The danger zone. Clement has not significantly changed the trends although goals for and against have both trended upwards. In defence, the central areas have been decimated by injury although the base numbers have always been low given he prefers two screening midfielders in front.

Despite spending on significant forward talent in the summer, the xG for has remained largely flat although Cyriel Dessers has weighed in with a goal flurry towards the end of the season. It is the uptick in goals conceded that will most concern the Rangers supporters.

We do not know what configuration Clement will go for in the Final, but with Conor Goldson struggling with persistent injury issues, and John Souttar lately lame, it may be patchwork at best.

Defensive pressing

Let us turn our attention to the off-the-ball pressing trends from both sides.

Celtic averages 69 per cent possession per match and average 135 pressure per game and 27 pressure regains. This has fallen off slightly as the season ends but the overall trends are for the team to be more aggressive and successful in their pressing. Both values are well up on Postecoglou’s side, which may surprise.

Over at Ibrox:

Rangers average 63 per cent possession and so are without the ball for longer than Celtic. Consequently, they average a higher volume of pressing (142 pressures) and regains (28). Not much in it, and reflective more of the nature of the average mismatched SPFL league game. The trend of note however is that whilst Celtic are trending to their seasonal average, Rangers have dipped below that at the most crucial moment of the season.

Can they regain their off-ball intensity for the cup final?

On-ball value and xG differential

StatsBomb’s proprietary on-ball value metric can be calculated at the team level. If we use this metric which considers all on-ball events and combines with the xG differential introduced above, we can gain an insight into the health of both teams as the season’s finale looms.

Celtic’s form took a dip in the new year as back-to-back defeats against Kilmarnock and Heart of Midlothian gave the Ibrox supporters hope. But, as injuries have cleared and the team is more settled, the OBV average has crept back up to be in line with the seasonal average and indeed exceeded it for the vital post-split matches. xG differential peaked in the autumn but is now a healthy 1.5 again at the business end.

For Rangers the trends are alarming. There was a concerted post-Beale bump in performances. Since hitting the top of the league however, the xG differential has fallen off a cliff. It is now trending at the dangerous 1.0 level whereby negative variance can be a factor.

Team OBV peaked in the 5-0 home win against Heart of Midlothian. Since then, however, it has trended downwards steadily as the season progressed and their lead became, eventually, an eight-point deficit.

Summary

It is almost as if for Rangers, hitting the summit of the league in February was “job done” as far as the players were concerned. Since then, the significant trends have been downwards and steeply so. Yes, injuries have worsened. But can they reverse the steady decline in performance levels with a depleted squad?

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For Celtic, it has been a gradual improvement over the season coinciding with their injuries clearing up at the right time in the campaign. All you can ask for is gradual but perceptible improvements and Rodgers is delivering that.

These games DO go to form in the main and Rangers have a large form mountain to climb to triumph at Hampden on Saturday.