With Manchester City without Rodri for the first time since his serious knee injury, this victory over Championship number eight Watford offers little insight into how they will fare in the long term.

Saturday’s trip to Newcastle should provide more insight into Pep Guardiola’s plan and which experienced player the manager trusts to replace the 28-year-old in midfield.

In this match, 19-year-old Nico O’Reilly, who would play in a competition for the first time, was chosen as number 6 of the vaunted Spaniard. Although he will not do so against bigger opponents, this fine performance has not done him any harm as an audition for the permanent selection for the match days.

The City machine was in threshing mode within five minutes with a ruthless Jérémy Doku finish. After Watford hesitated at the back, James McAtee, on the left byline, turned and fed a pass to Jack Grealish. The number 10 played the ball to the Belgian, who smoothly outmanoeuvred James Morris and beat Jonathan Bond, the visiting goalkeeper.

In pre-season, Guardiola joked about the 6ft 4in O’Reilly, saying it was a change from City’s smaller youth players Rico Lewis and Oscar Bobb. Rodri is also statuesque, and a simple touch from O’Reilly to Lewis at Watford also evoked the injured man with a cuteness that caught the right-back off guard.

A quick tackle from O’Reilly on Imran Louza, which left No. 10 in a difficult position and City on the attack, will have pleased Guardiola, but an attempted dribble that gave the ball away was less satisfying, a mortal sin in the manager’s eyes.

Guardiola made nine changes from Sunday’s combative 2-2 draw with Arsenal, with Kaden Braithwaite becoming the club’s third-youngest debutant at 16 years and 229 days. He was pushed aside by Kwadwo Baah on his way to scoring an apparent equaliser before a foul was called by referee David Webb. A 50-50 call, this might not have been given outside the City penalty area, so the youngster was lucky and the Hornets the reverse.

However, Phil Foden’s foul when he picked up a pass from Grealish was a foul. The two were standing by the free-kick, but Foden fired the ball from the edge of the box against the Watford wall.

Ten minutes before half-time, Nunes fared much better. Lewis, drifting inside, pinged the ball to McAtee, who brought it back. The defender tapped it to Matheus Nunes who, sensing the yellow shirts around him lingering, moved the ball to his left and then passed Bond to his right from 20 yards.

Guardiola’s expertly drilled unit had its opponents where he wanted them: on the wrong side of a deficit and facing a relentless blue attack. This was the pattern of the first half until Webb wasted time. But had the unmarked Vakoun Bayo not missed an easy header moments earlier, the managers’ team talk would have been very different.

Savinho, whose soft-shoe shuffle and turn inside created Erling Haaland’s opening goal against Arsenal, was given the second 45 minutes when Doku was substituted. Link play with the Brazilian led to a corner, but from it McAtee fired a pass to Lewis, who had miscontrolled it, and Watford broke upfield, Nunes doing well to sprint back to kill the threat.

Lewis then weighed the ball perfectly for McAtee who raced into the visitors’ area, but his touch was clumsy and Bond collected. City got going. Two twisting efforts from Foden claimed two corners. A Savinho laser rippled the side-netting. Down the left, Grealish probed and teased a backline as determined as the Gunners had been in their second half here.

It was no relief for Tom Cleverley’s men to see Ruben Dias, Bernardo Silva and Mateo Kovacic warming up, but the corner they claimed was. The ball was begging to be smashed home as it fell temptingly onto Louza’s boot, but his volley was wild.

Seconds later, City were hot on Watford’s heels again as Nunes and Lewis went close and Tom Ince cleared Savinho off the line. It was all hectic.

A worried Guardiola was almost disappointed by the near misses as Watford broke again, with Baah firing narrowly wide of Stefan Ortega’s goal. At the other end, Savinho closed in, hitting Bond’s right post and eliciting oohs from a crowd that had not had enough players.

The evening was over for O’Reilly. Jacob Wright replaced him, and shortly after Braithwaite. Josko Gvardiol took his place at left back.

Grealish, from close range, smashed the ball onto Bond’s chest. The wide man should have scored, but despite Ince’s curling shot from distance it proved inconsequential.