Sporting's 2-1 win against Portimonense in the third round of the Taca de Portugal (their FA Cup equivalent) on Friday, October 18, doesn't, on the face of it, look like much of a marker for Ruben Amorim's year so far. But dig a little deeper, and there is a relevance to that game.

The 2-1 victory is one of only four occasions this season in 23 matches that Amorim hasn't made all five substitutions available to him. It is also the last time he failed to empty his bench and change half of his outfield players during a match. Since then, Sporting and Manchester United have used 60 subs out of a possible 60.

This season, Amorim has used 111 out of 115 substitutes. He used four of the five available in that game against Portimonense, in the Champions League against PSV Eindhoven and Lille, and in a league victory against Porto. At United, he has used all his subs in his six games in charge.

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That presents an interesting contrast with the man who will stand just down the touchline for the second time in six weeks on Sunday. Pep Guardiola made just two substitutes on Wednesday, despite City losing in Turin to Juventus. In 24 matches this season, the Catalan has made 75 out of a possible 120 substitutes. It is quite the contrast.

Guardiola's reluctant use of the bench has often been a talking point at City. He is only likely to make the full quota of five permitted changes if his team are comfortable, and has done so only five times this season. Four of those came in home wins against Ipswich (4-1), Brentford (2-1), Sparta Prague (5-0) and Nottingham Forest (3-0). The other was in a Carabao Cup defeat to Tottenham that Guardiola had little time for.

Injuries have also affected the use of subs this season. Guardiola made only two changes on Wednesday, but Mateo Kovacic and Phil Foden were only just back from injury, and the other subs available were either goalkeepers or academy players. It was the same story at Crystal Palace four days earlier.

Guardiola might have more depth on derby day if Kovacic and Foden are ready to play some part, but his resources remain stretched. The perils of going into a season with a small squad have hit hard this season, with injuries a constant issue, especially during a run of one win in 10 games.

That run includes the 4-1 defeat to Amorim's Sporting, a match that helps to explain the use of the subs bench for both. Sporting led 3-1 after 49 minutes, but Amorim made three changes before Guardiola, with City chasing the game, turned to his bench on 77 minutes.

It might be a similar story at the Etihad on Sunday. Amorim will likely use all five subs. He has accepted United's players don't have the physicality to sustain his demands, which has led to changes, but he also has a squad with only one senior absentee (Luke Shaw). His options are much greater than Guardiola's.

Amorim's changes are also influencing matches and giving United fresh legs. His introducing of Rasmus Hojlund, Mason Mount, and Antony on Thursday, with United losing to Viktoria Plzen, helped swing the game back in their favour.

How both bosses use their bench on Sunday, especially in the second half, will be fascinating. Few will dispute that City have the stronger starting XI, but for once, injuries and form have conspired to offer United more options in reserve. That could be crucial when Amorim rings the changes and whether Guardiola will stick with what he's got or look to respond.