When Manchester City examine the final Premier League table, they will wince at the unacceptable gap between themselves and the top two sides. It will be impossible for them to ignore their need to improve. Pep Guardiola is unlikely to treat himself to a summer of relaxation after enduring the first trophyless season of his managerial career, even though it ended with City securing automatic passage into the group stage of the Champions League thanks to a brutal evisceration of Watford.

They ruined Walter Mazzarri’s farewell in classic Guardiola style, with killer passing, kaleidoscopic movement and wondrous skill, but a squad containing such a vast array of talents must aim higher than third place. They must produce football of this quality on a weekly basis and prove that they possess the durability required to handle the pressure of a title race. “We finished 15 points behind Chelsea,” Guardiola said. “We have to play better.”

For City’s idealistic manager, that means greater refinement and poise. Despite facing criticism during his first year in England, Guardiola made it clear that he will not betray his principles. “We are going to try to play better,” he said. “It is the only way to improve. Not tackles. Not running harder. To play better.”

It seemed that City might win the title at a canter after the Guardiola era opened with six consecutive triumphs in the league, but that start turned out to be a deception once the first cracks in this gifted but flawed team appeared and momentum slipped away. They have underachieved in a manner that their owners cannot have envisaged when they tempted the most coveted coach in Europe to Manchester and there will be changes this summer. Pablo Zabaleta will leave on a free transfer and the defender could be followed by Willy Caballero, Bacary Sagna, Gaël Clichy, Jesús Navas and Yaya Touré, who shared a warm embrace with Guardiola after the final whistle. “I said thank you for everything,” Guardiola said. “It was an outstanding performance.”

Players such as Touré represent City’s past. While Guardiola insisted that he will not sell Sergio Agüero, whose first-half double lifted his tally to 32 goals in all competitions, this squad needs refreshing. It felt unsatisfactory that City arrived at Vicarage Road still unsure of a top-four place.

On a warm afternoon in Hertfordshire, however, they could not have asked for more obliging hosts than Watford, who lined up with a midfielder in central defence because of injury problems, a callow teenager at left-back and two goalkeepers on the bench. If this was Mazzarri’s attempt to make his bosses row back on their decision not to give him a new contract, it backfired badly. Watford’s players hardly broke sweat and Mazzarri did little to endear himself to the home fans by naming Troy Deeney among the substitutes. The Italian received loud boos on the one occasion when he stepped into his technical area.

Watford’s huddle moments before kick-off fooled nobody. Perhaps they were discussing their holiday plans. It was a disingenuous attempt to prove their commitment to the cause and it looked utterly ridiculous once the game began.

City let their sublime football do the talking and Watford held out for five minutes, Kevin de Bruyne swinging in a corner from the right for Vincent Kompany to thump a header past Heurelho Gomes. The marking was pathetic.

José Holebas had already cleared a Gabriel Jesus header off the line in the first minute and Watford’s response to falling behind was feeble. Valon Behrami was hopelessly out of depth alongside Holebas in defence and Brandon Mason, making his first league start, was powerless to stop Jesus on the left. M’Baye Niang’s failure to track back hardly helped the 19-year-old left-back, whose frailties were horribly exposed when Fernandinho charged past him on the way to scoring City’s fourth goal four minutes before half-time.

Mazzarri’s demise will not be mourned in these parts, although he defended his work this season. “When we reached 40 points, we were 10th,” he said. “A less experienced manager would have done worse.”

Yet this was one of the most inept performances from any team in any league this season. Watford offered no resistance and De Bruyne had an embarrassing amount of space to glide into when he turned away from Abdoulaye Doucouré in the 23rd minute before releasing Agüero, who swept a low finish beyond Gomes.

David Silva, as majestic as ever, created City’s third goal with a pass that sent Leroy Sané tearing away from Daryl Janmaat. Sané unselfishly squared for Agüero to tap home his second.

Agüero had chances to complete his hat-trick in the second half, but the Argentinian did play a pivotal role in City’s fifth. Watford had briefly raised their level after half-time, with Stefano Okaka forcing Caballero to make his only save with a powerful drive from the left and Niang shooting wide from a tight angle, yet City soon quelled any suggestions of an unlikely uprising. Agüero burst past Mason on the right and his chip found Jesus, who controlled expertly before scooping the ball over Gomes.

To their credit, Watford’s supporters stuck around to applaud their team during the lap of honour after the final whistle. “You give us a bit of shit from time to time,” Deeney said. “But we deserve it.”