If Antonio Conte was shocked this week by Diego Costa’s reaction to a transfer offer from China, the manager enjoyed a pleasant surprise here thanks to Marcos Alonso. The Spanish wing-back struck two goals as Chelsea made light of their top scorer’s absence and inflicted a heavy defeat on the team whose title they intend to take. A later goal by Pedro emphasised the point.

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The line from Conte is that Costa was unavailable here because of pain in his back but the whiff around the absence of the Premier League’s leading scorer highlighted what a pain in the neck China risks becoming to English clubs. But Chelsea’s immediate task here was to respond to domestic threats. They did so emphatically, even if Leicester’s resistance was feeble.

Chelsea’s winning streak had been brought to a shuddering end in their last league match by Tottenham Hotspur, who moved even closer to the leaders by mauling West Bromwich Albion in Saturday’s early game. Chelsea restored their lead at the top of the table by outclassing the flagging champions.

Without Costa Chelsea deployed Willian in a fluid front three, just as he had done on Boxing Day, when Chelsea were deprived of the striker by suspension but still proved too strong for Bournemouth, winning 3-0. Here, too, the relentless movement and mischief-making of the Brazilian, Pedro and Eden Hazard ensured they had a fearsome attack. Leicester were toothless. They, at least, need not worry about anyone bidding for their main striker, not with Jamie Vardy in this form.

In fairness to Leicester they were shorn of more players than Chelsea, with Riyad Mahrez, Islam Slimani and Daniel Amartey at the Africa Cup of Nations. But Nigeria’s failure to qualify for the tournament in Gabon meant that Ahmed Musa and Wilfred Ndidi were available here. Ndidi’s first league appearance since his £15m purchase from Genk provided a handy opportunity to compare him with N’Golo Kanté, the man who left a void in Leicester’s midfield when lured away by Chelsea’s riches after helping City win last season’s title.

The Frenchman was given a warm welcome back by Leicester fans, who used to refer him affectionately as “the Kanté twins” because his dynamism made him akin to two players. Claudio Ranieri sought to outnumber him here and counter Chelsea’s other threats by switching to a 3-5-2 formation, with Ndidi, Nampalys Mendy and Danny Drinkwater deployed in the middle. Ranieri later explained that the ploy had worked in the second half of the sides’ meeting at Stamford Bridge this season and insisted that it worked well here, too, even though the result was another 3-0 defeat.

In truth, Leicester rarely bothered Chelsea after the second minute, when Courtois had to make a smart save from Musa. Then Chelsea flexed their muscles and Leicester wilted.

In the sixth minute a cross from the right by Azpilicueta triggered chaos in Leicester’s three-man central defence, where Hazard summoned the poise to apply a telling touch. While all around him panicked, the Belgian tamed a loose ball and rolled it calmly to Alonso, who scored with an impeccable curling finish of which Costa would have been proud.

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Chelsea controlled proceedings after that. Leicester did not seem at ease with their new system, their discomfort exacerbated by the visitors’ slickness. Not until the 36th minute did the hosts so much as fluster the Chelsea defence again but no Leicester player was on hand to take advantage of a vicious cross by Vardy, which Courtois intercepted with difficulty.

That faint chance was Leicester’s clearest sight of an equaliser before the break. Ranieri decided against tinkering during the interval, seemingly confident that his side could creep back into the game so long as they did not let Chelsea go farther ahead. Six minutes into the second half that theory was binned. And Alonso’s stock soared even higher.

After a foul by Christian Fuchs on Willian, the latter floated a free-kick into the Leicester box. Again the attempted clearance was flawed and when the ball broke to Alonso at the edge of the area, he trapped it with one touch before lashing it into the net, a deflection off Wes Morgan helping to confound Kasper Schmeichel.

Facebook Twitter Pinterest Marcos Alonso, with the help of a deflection off Wes Morgan, scores the second Chelsea goal. Photograph: Tom Jenkins for the Observer

That was Ranieri’s cue to scrap his original plan and revert to a more familiar 4-4-2 formation, with Shinji Okazaki coming on for Robert Huth. But getting back into the game would entail subduing visitors who were starting to revel. Gary Cahill exemplified that in the 61st minute with an overhead kick at goal. Fuchs’s block foiled a splendid effort but no one could get in the way of a spectacular 18-yard volley by Alonso moments later that whizzed inches past the far post.

Chelsea gave themselves further cause for celebration with a third goal that resembled a party trick. In one movement at the edge of the area Pedro spun and back-heeled the ball to the overlapping Willian, who clipped it over Schmeichel and back into the path of Pedro, who nodded into the empty net.