The much-changed Blues looked disjointed in the opening exchanges, before play was temporarily suspended due to a medical emergency after a supporter was taken ill, with members of both teams' medical staff assisting the paramedics in the stands. That gave Tuchel the chance to drill some urgency into his players, and Mason Mount grabbed the lead from close range after he had struck the woodwork minutes earlier.
The hosts hit back through Emmanuel Dennis who found the corner to make it 1-1, but champions find a way to win when they're up against it, and that's exactly what Chelsea did. Mount was again involved, this time teeing up Hakim Ziyech, who produced a clinical finish to break Watford hearts.
The biggest positive of the night has to be the urgent response of the medical teams, who swiftly attended to the emergency in the stands.
Chelsea early on had no answer to their unrelenting hosts, who had their opponents playing themselves into all kinds of trouble. The Blues were second-best all over the park for large spells, and they can't risk too many nights like this one if they're to lift silverware in May.
Thomas Tuchel, 6 -- Six changes, and it showed. Even the best sides need to know when not to play out from the back, but his team failed to recognise that. Managers are judged on results though, and ultimately Chelsea delivered, helped by some smart substitutions.
Player ratings (1-10; 10 = best, players introduced after 70 minutes get no rating)
GK Edouard Mendy, 7 -- The Chelsea goalkeeper probably wouldn't have anticipated a touch inside the first minute, but he was his side's busiest player from the off and was forced into a flying save to keep the Blues level before denying Danny Rose from close range.
DF Trevoh Chalobah, 7 -- The 22-year-old provided some vital composure in trying circumstances and was in the right place at the right time to deflect a dangerous swept effort wide of goal with Vicarage Road ready to erupt. A decent display prior to his departure.
DF Andreas Christensen, 5 -- Required early treatment following a painful clash with Joshua King as Chelsea struggled to escape their own half. Crucially on hand to smash the ball away to safety after Mendy had denied Rose, but he always looked vulnerable.
DF Antonio Rudiger, 5 -- Miscued clearances all over the park. He was beaten too easily by Dennis, who fired through his legs and into the corner, but he started the attack that made it 2-1 with a smart, scooped pass out to the left.
MF Cesar Azpilicueta, 5 -- Survived a heart-in-throat moment under pressure from Joao Pedro when a whipped cross from Kiko Femenia cannoned off a knee and had to be beaten away by Mendy. Struggled against a Watford attack that was packed with pace.
MF Ruben Loftus-Cheek, 6 -- The athletic midfielder made little impact in a first half where, apart from the goal, Chelsea struggled to string a decent run of passes together. Much, much better after the break, and he smartly unleashed Christian Pulisic after a powerful burst forward.
MF Saul Niguez, 4 -- He was practically invisible in the middle and his only contribution of note was when he was hauled off at the interval having seen yellow for a pull on Dennis.
MF Marcos Alonso, 8 -- A tough night defensively, but he was the architect of the opening goal when he slotted in Kai Havertz who squared for 1-0. More importantly, he helped alert the medical staff to the fan in need of urgent attention, and the rest pales into insignificance.
FW Mason Mount, 9 -- Came within a couple of inches of opening the scoring against the run of play when his sweetly-struck effort from a narrow angle rattled the left post. That was before he fired Chelsea ahead from close range, and he crossed for Ziyech to snatch all three points.
FW Christian Pulisic, 6 -- He did his best to provide an outlet but with little success in the opening 45 minutes. Had a lot more joy after the break but lost his footing at the vital moment when he looked destined to score. Ended the night at right wing back.
FW Kai Havertz, 7 -- Completely isolated for about half an hour, but he grabbed himself an assist when he selflessly put the ball on a plate for Mount to drill home. He had the hosts scrambling after the break when he shook his marker, but his teasing low pass was well-defended.
DF Thiago Silva, 6 -- Replaced Niguez at the break with Chalobah pushing into midfield, but he didn't actually do much until the final moments, when he helped his side survive a wave of late pressure.
MF Hakim Ziyech, 7 -- Replaced the injured Chalobah on the hour-mark with Tuchel forced to shuffle the pack again. Picked up a yellow card less than five minutes later and his passing was less than impressive, but he stole the headlines when he emphatically crashed home.
FW Romelu Lukaku, 6 -- Tasked with rescuing a dire situation when he replaced Azpilicueta as Tuchel went all in, but it was in his own box where he made his biggest contribution, relieving the pressure with a towering header.