It's one of the few mistakes Thomas Tuchel has made in his short but transformative time as Chelsea manager.

One he has quickly rectified.

Just two days into the job as Frank Lampard’s midseason replacement, Tuchel picked his first Chelsea lineup for a Premier League game against Wolverhampton in late January and, perhaps understandably, he chose to drop Mason Mount.

New to English soccer, Tuchel chose to side mostly with experience and players he knew — sturdy defenders Antonio Rudiger and Cesar Azpilicueta, a fellow German in Kai Havertz, a 34-year-old striker in Olivier Giroud.

Mount, a 22-year-old midfielder halfway through only his second season at Chelsea, didn’t fit the bill.

Fast forward three months and the situation couldn’t be more different. Mount is a player Tuchel cannot do without, a lock in the team — and not a stretch to say maybe Chelsea’s most important player.

“He has the full package,” Tuchel said of the playmaker after Chelsea beat Fulham 2-0 in the league on Saturday, when he chose to give some first-choice players a breather between the two legs against Real Madrid in the Champions League semifinals.

Not Mount, though, a player so integral to the team — with his vision, his set-pieces, his ability to drive forward with the ball and his pressing — that apparently he couldn’t be left out.

For someone who was first spotted by Chelsea scouts at the age of 6 and then signed by the club at 9, Mount has come a long way in a short time.

Three seasons ago, he was on loan at Vitesse Arnhem, Chelsea’s Dutch feeder club.

Two seasons ago, he was on loan at Derby, a second-division English club managed at the time by Lampard. Mount was clearly technically gifted, with a cultured right foot and strong work ethic, but was hardly someone who grabbed the headlines.

Last season, he followed Lampard back to Chelsea and was part of the senior squad for the first time, one of the young players regularly used while the club was hindered by a transfer embargo.

Unsurprisingly, given his role as an attacking midfielder — like Lampard was — Mount was regarded as something of a pet project for his manager.

But he proved his worth by the end of the season, scoring and assisting in the final-day 2-0 win over Wolves that secured Champions League qualification for Chelsea, and Mount continued to be a regular under Lampard even after the club’s nearly $300 million spending spree in the offseason.

Tuchel quickly learned Mount is the real thing.

“He has big self-confidence, in a very, very positive way,” Tuchel said. “It’s a pleasure to have him, be at his side and guide him and support him.

“It’s impressive, he’s not affected by all the praise. This is maybe the most important part. He seems to be a guy who likes the sport and feels an obligation to make the most out of it. And he wants it absolutely badly, so it’s a top mix.”

Mount is delivering on the international stage, too, having started the last eight matches for England and seemingly a shoo-in for Gareth Southgate’s first-choice team for the European Championship in June. He is fronting a new generation of young players, a list that includes Phil Foden, Jadon Sancho and best friend Declan Rice.

Southgate has long been a fan of Mount, even inviting the midfielder to train with the senior team for a week ahead of the 2018 World Cup.

“He is an exceptional player,” Southgate said. “He finds space intelligently, he manipulates the ball very well, he creates chances and scores goals.”

Tuchel has discovered this. In the first leg against Madrid, from which Chelsea came away in the ascendancy after a 1-1 draw in Spain, Mount finished with a superior pass completion rate (96%) than Madrid’s superstar midfielders Toni Kroos (92.4%) and Luka Modric (86.9%).

Against Fulham, he came off holding his lower back after an awkward fall — but not before producing the highlight of the match when he brought down a long ball from Thiago Silva with a deft touch before slipping in Havertz for a goal with his next touch.

On Tuesday, Tuchel declared Mount fit to play against Madrid in the second leg.

In 101 games for Chelsea, Mount has 16 goals and 14 assists — superior numbers than Lampard, the club’s record scorer, at that stage.

But it's not just his attacking output. He is a leader of an intense press higher up the field, which is key to Chelsea being "an annoying, horrible team" to play against, as defender Andreas Christensen said on Monday.

Don’t expect Tuchel to drop Mount again.

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Steve Douglas is at https://twitter.com/sdouglas80

Antonio Rudiger

Mason Mount changing perceptions in his lead role at Chelsea