As if things were not bad enough for Tottenham, with their team trailing 2-0 against Chelsea and reduced to 10 men following Son Heung-min’s rush of blood, it was left to a moronic element of their support to take matters down into the gutter.

It was midway through the second half when Antonio Rüdiger, over in a corner of the stadium, had a racist chant directed at him. The Chelsea defender knew what he had heard and TV pictures would show him indicating it involved a monkey sound.

César Azpilicueta, the Chelsea captain, reported it to the referee, Anthony Taylor, who then went over to inform the officials on the touchline. Cue an announcement over the Tannoy that “racist behaviour from spectators is interfering with the game”. It needed to stop. The announcement would be made on two further occasions in line with Premier League protocols.

It was the latest racist incident to blight football in England and so utterly depressing. This ought to have been the day to salute Frank Lampard’s outfoxing of his old mentor, José Mourinho; to lavish praise on Chelsea for how they pulled themselves out of a slump to maintain a hold on fourth place; to revel in a host of stand-out performances from those in blue. Instead we were left to ask once again what had happened; how, in one of the world’s greatest and most multicultural cities, it had come to this.

Rüdiger had been involved in the Son red card on 62 minutes, falling slightly theatrically after the Spurs winger had pushed his studs up and into his rib cage from a position on the ground, and that possibly brought the red mist down in the stands. It does not take much. Curiously the replay of the incident was shown on the big screen.

It was a moment of recklessness from Son, which invited the VAR official to make the decision, and he is now looking at a three-match suspension. Tottenham will investigate the abuse that was hurled at Rüdiger shortly afterwards and the person or people that are identified can expect to be banned from stadiums for rather longer.

Spurs could not get off the field quickly enough at the end and that was down to their display, which was strangely uncoordinated and epitomised by the rush of blood from the goalkeeper, Paulo Gazzaniga, in first‑half stoppage time. What was going through his mind as he charged off his line to deal with a long punt forward?

Gazzaniga looked certain to catch the ball, which would have been a good idea, or he might have left it and allowed it to run out, which would also have worked. Instead he launched himself into a side-on kung-fu kick, his boot dangerously high, missing the ball and clattering into Marcos Alonso. It was a clear penalty, even if it took the intervention of VAR to award it. Willian converted for his second of the game and Chelsea could cruise to a victory that was fully deserved.

Lampard had felt the need to try something different after four defeats in five league games and he switched to a 3-4-3 system which he had previously started with at Lille on 2 October. It was a bold move and it paid off handsomely.

N’Golo Kanté and Mateo Kovacic dominated in central midfield and, with Tammy Abraham holding the ball up in the No 9 role, Chelsea controlled possession for decisive spells while carrying an attacking threat via the wing-backs. As for Willian, a stellar performance was summed up at the end when he controlled a high ball with a velvet touch before pivoting and playing a lovely pass. In the technical area Lampard blew out his cheeks in wonder.

The opening goal owed much to Lampard’s plan to target the Spurs right-back, Serge Aurier, who likes to play high and, consequently, leaves spaces behind. Rüdiger tested him with the first of many long diagonals in the 12th minute and Aurier could only head behind for a corner, from which Chelsea went ahead. Lucas Moura and Aurier dozed as Willian worked it short to Kovacic, got the ball back, glided away from Aurier and bent a beautiful shot into the far corner.

Spurs were dreadful in the first half, lacking coherence and authority in midfield, and they had nothing more than flickers in the final third. Harry Kane lifted high from Moussa Sissoko’s cross and Son did likewise from a tight angle beyond the far post from a Kane cross. At the other end Abraham could not get enough purchase on a free-ish header; the excellent Mason Mount was crowded out after a positive dart and Abraham failed to control when well placed. Then came Gazzaniga’s aberration. There were loud boos from the home fans on the half‑time whistle.

Mourinho took Eric Dier off for Christian Eriksen and adjusted to 3-4-2-1 for the second half, with Lucas at left wing-back. Things would get worse for the home team. Their frustration had been simmering – witness Dele Alli’s clash with Kovacic towards the end of the first half, which ended with both players being booked.

Alli could get nothing going in attacking terms, although he was not the only Spurs player to fail on this account. Lampard’s system served to stymie the runners in behind. And it all bubbled over when Son kicked out at Rüdiger.

It signalled the end of the story on the pitch; sadly, it was just beginning off it. Kepa Arrizabalaga had a missile thrown at him from the South Stand, which narrowly missed him, but it was the abuse of Rüdiger that would leave the bitterest of tastes.