Unai Emery had to tear up the plan. Aston Villa had started this game in a compact 4-2-2-2 formation but they were desperate for much of the first hour. Their passing was lethargic, they had no width, they were 1-0 down to a dominant Chelsea and it seemed the story was going to centre on Emery’s decision not to start Ollie Watkins up front.

Yet few managers are as effective as Emery when it comes to altering the flow with a few smart tweaks. The Spaniard never stops plotting, never stops hunting for weaknesses in his opponents, and perhaps it was not a surprise when Villa’s tactical mastermind found a way to ensure that the travelling fans were belting out songs about winning the league at full time.

Key to it all was Emery having the clarity of mind to change Villa’s shape and alter the flow with a triple substitution early in the second half. Do not mistake it for luck. Villa retrieving 18 points from losing positions this season is no fluke. They have a group full of leaders, as Emery was quick to point out after an 11th consecutive win in all competitions, and it helps that they have players who are ready to make an impact when thrown on from the bench.

The feel was different once Villa had Amadou Onana’s muscle in midfield and Jadon Sancho looking to prove a point against the team who decided against signing the winger in the summer. Chelsea, on the other hand, failed to react. Their lead was cancelled out by Watkins equalising five minutes after coming on and their problems increased when the striker lifted Villa three points below Arsenal at the top of the league after rising to head in the winner in the 84th minute.

Where does this end? Emery continues to play down talk of a title challenge but Villa, who visit Arsenal on Tuesday, are building serious momentum. Crucially, they do not know when they are beaten. They have fallen behind in each of their past six away games but have recovered to win five of them.

What Chelsea, who have slipped to fifth place after one win in six league games, would give for some of that knowhow. Unfortunately for Enzo Maresca, though, his young side’s habit of playing in fits and starts is why they are still not realistic challengers. “We need to manage the game better,” Chelsea’s head coach said. “We were completely in control – and this has happened before.”

Indeed, Chelsea’s collapse after Villa’s alterations was no one-off. They have dropped 11 points from winning positions at home this season and it was a worry for Maresca – who was forced to watch from the back of the press box after being hit with a touchline ban – that none of his substitutes were able to make an impact.

The lack of leadership was stark. Chelsea lost their shape after Villa equalised, while another headache for Maresca was Cole Palmer not hiding his displeasure at being substituted in the 72nd minute. Palmer, who is still feeling his way back from a groin injury, flung his arms in the air when his number went up and briefly disappeared down the tunnel after making way for Estêvão Willian.

In truth, though, Chelsea’s No 10 did not have one of his better games. Palmer was lively early on, whistling a volley wide, but he was outshone by Morgan Rogers in the second half. Villa’s star man made Watkins’s first goal and there were times when Chelsea could not get close to the England international.

It was some turnaround. Chelsea were excellent during the first half. They neutralised Villa as an attacking force, were full of aggression and went ahead when João Pedro turned a Reece James corner past Emi Martínez in the 37th minute.

Yet Chelsea could not push on. They faded after being denied a penalty when Ian Maatsen handled a cross from Alejandro Garnacho. Emery, who could have made his changes during the interval, came to the fore. Villa were far more menacing with Watkins leading the line. He almost made a goal for Boubacar Kamara, who was thwarted by Robert Sánchez, and he equalised when Onana pounced on a stray pass from Benoît Badiashile.

Onana was alert, shovelling the ball to Rogers, who sent Watkins through to bundle an effort past Sánchez in the 63rd minute after the striker’s first shot was saved by the goalkeeper. Now Villa believed. Watkins and Maatsen forced saves from Sánchez, who was relieved when a Rogers free-kick flew inches over.

Chelsea offered little at 1-1. Villa looked the likelier winners. They kept pushing and their moment arrived when an error from Malo Gusto, who had come on at left-back for the hamstrung Marc Cucurella, led to Watkins sparking dreams of glory by rising highest to head in a Youri Tielemans corner.