The Spain striker, rumoured to be the target of a big-money bid from Chinese Super League outfit Tianjin Quanjian, was dropped after a reported bust-up with manager Antonio Conte and fitness coach Julio Tous in training this week.
Conte spoke ahead of the match to confirm the 28-year-old – the division's joint-top scorer – had suffered a back injury, though the Italian said nothing regarding any falling out and was soon celebrating an opener at the King Power Stadium.
Alonso provided a neat finish from close range to hand Chelsea – for whom Michy Batshuayi remained on the bench – a sixth-minute lead, showing a spirited response to the earlier 4-0 wins of London title rivals Tottenham and Arsenal.
In a fixture pitching the title holders against the leaders, it was the latter who looked more worthy of the mantle of champions and Alonso scored again six minutes into the second half, before Pedro's late header wrapped up a third win over Leicester this season.
The visitors, beaten by Spurs last time out in the league to end their 13-match top-flight winning streak, were ahead early on, with neat build-up play leading to a mishit shot from Pedro, which fell kindly for Eden Hazard to selflessly lay on a simple finish for Alonso.
Leicester's strong home form, which had yielded 18 of their 21 points prior to kick-off, marked them down as dangerous opponents, but Ranieri's men – deprived of the services of Africa Cup of Nations-bound trio Riyad Mahrez, Islam Slimani and Daniel Amartey – lacked fluency against his former club.
Ben Chilwell's header from Marc Albrighton's right-wing cross was well wide in the 17th minute, while Wilfred Ndidi sliced off target with an ambitious 20-yard attempt.
A Jamie Vardy cross-cum-shot forced Thibaut Courtois into action on the half-hour mark and Pedro blazed over the crossbar for the away side following a well-worked set-piece routine just before the break.
Any resolve on Leicester's part to make it a difficult evening for Chelsea vanished soon after play resumed, with Alonso collecting Willian's partially cleared free-kick to whip in a shot that took a deflection off Wes Morgan and beat Kasper Schmeichel.
Centre-back Gary Cahill showed the technique of a seasoned striker to direct an acrobatic effort towards goal, but Morgan was on hand to block six yards out.
Alonso almost capped a superb solo display with a hat-trick when his well-executed volley bounced narrowly wide of Schmeichel's left-hand upright, with Conte's men looking to put the match well beyond Leicester's reach.
And they did just that in the 71st minute as Pedro headed into an unguarded net after Willian's pass had reared up off Schmeichel, with the visitors leaving their White Hart Lane frustration behind them to reassert their position as clear favourites to win the Premier League.