LONDON -- There is something about Kai Havertz that makes it impossible to gauge the Chelsea forward's mood. Whether he is good, bad or indifferent, the 21-year-old carries the same unassuming expression like an impenetrable mask and it can hint at an air of indifference, which has not helped him at times during a difficult first season at Stamford Bridge.

To suggest he has struggled since his £62 million transfer from Bayer Leverkusen would be an understatement, but against Everton on Monday, the Germany international finally showed us why Chelsea were so determined to win the race to sign him.

Havertz has still only scored one Premier League goal this season -- against Southampton last October -- but it wasn't for lack of trying that his barren run continues after Chelsea's 2-0 victory over Carlo Ancelotti's team that puts them firmly in control of their pursuit of Champions League qualification. Havertz was the central figure for Thomas Tuchel's team, with the player directly influencing both goals, and it would have been 3-0 had his second-half goal not been correctly ruled out due to a handball.

A heavy deflection off Ben Godfrey, following Havertz's shot from Marcos Alonso's 31st-minute pass, ensured that the opening goal was attributed to the Everton defender rather than Havertz, despite the best efforts of the Stamford Bridge announcer to give the goal to the Chelsea man. And a reckless challenge by Everton goalkeeper Jordan Pickford on Havertz led to a 65th-minute penalty that Jorginho scored to assure Chelsea of the three points.

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Havertz, having made both goals, smiled briefly on both occasions before returning to his usual look of nonchalance. Too often this season, he has given the same expression after poor performances and missed chances, but this time at least, he had a spring in his step and his shoulders did not appear to be carrying the weight of the world on them.

"I am very pleased with Kai," Tuchel said. "It was the trust we gave him and he used the trust we gave him. He is a player who has all the ability to be a dominant figure in [attack] and he stepped up."

One impressive performance, against a top-six rival, should not be regarded as indisputable evidence that Havertz has finally arrived as a Chelsea player, but it is a positive sign and another example of the impact that Tuchel has had as coach since replacing Frank Lampard in January.

Two months on, Chelsea are undefeated in 11 games under the former Borussia Dortmund and Paris Saint-Germain coach and in the top four. Tuchel has turned Chelsea around, but he has also transformed players individually, too.

Lampard, to his cost, could not get any kind of form out of Havertz. He seemed to have no idea how best to use the languid youngster, and Havertz flopped in whichever role Lampard asked him to perform.

Tuchel believes his compatriot's best position is in the forward line and he deployed him alongside Timo Werner as a split striker against Everton. Werner, whose performances have also improved under Tuchel despite his ongoing search for goals, offers boundless energy and work rate while Havertz prefers to glide around the pitch, looking for pockets of space.

But while their qualities failed to deliver for Lampard, they knitted together against the Toffees.

Werner will get fans on his side because of his work rate, but Havertz needs to have something tangible to show for his efforts because his style -- half Dimitar Berbatov and half Mesut Ozil -- could be misconstrued as laziness and lack of commitment when things aren't going well. Tuchel's demanding persona on the touchline may be just what is required to keep Havertz on track because the coach is quick to bark out orders at any player he perceives to be underperforming, but when Havertz influences games as he did on this occasion, any coach will forgive what appears to be a half-hearted approach to the game.

In time, Tuchel's abrasiveness may work against him if his players grow tired of the constant hectoring, but his style is working at this moment in time and Chelsea are getting results. Defensively, they have kept clean sheets in all five Premier League home games under the German.

Havertz, meanwhile, is not the only player to be benefiting from Tuchel's tough love. Werner has raised his game, while Alonso, Andreas Christensen and Mason Mount have also kicked on under the new coach.

It is perhaps why Chelsea are averaging 2.33 points per game under Tuchel compared to 1.53 under Lampard. They are enjoying a greater share of possession in games, registering more passes and winning more tackles with Tuchel than Lampard, so the progress is clear. And Chelsea now have a cushion in fourth, one that enables them to start looking up the table and target Leicester City and Manchester United, who are three and four points ahead of them in third and second respectively.

When Tuchel arrived in January, he was tasked with saving Chelsea's season, but he has already gone beyond base camp and is now improving the collective and the individuals. And getting the best from Havertz shows that Tuchel really is making a difference.

"You see the quality of the players and the club," Tuchel said. "It is a pleasure to be on the sideline and work with the team. Everybody in the club is doing everything to compete at this level."