Liverpool were four goals up just two minutes later, as Salah netted a sublime goal to match his effort against Manchester City after showing superb skill to create space for himself in the box and fire home. Firmino capped off a joyous game for the visitors by completing his hat trick in the dying second, and Watford were never in the game at all.
Liverpool enjoyed complete domination for pretty much the entire game, moving the ball around with precision to ensure they hurt Watford rather than just keeping hold of possession. They also looked comfortable defensively, with the Hornets unable to do much with the ball even when they did have it.
While Liverpool controlled the game from the beginning, there were periods where they were clinical to ensure the scoreline matched the game's flow. The hosts were truly poor, but Liverpool could only beat what was in front of them and continued pushing until the very end.
We have to be very picky here, but there were a few signs of sloppiness and complacency around the 20-minute mark when the score was still 1-0, though they were removed relatively quickly. Watford also had the occasional moment near the end, but the game was already gone by then.
Manager rating out of 10
9 -- Jurgen Klopp didn't have every player available and complained about the fixture timing again, but that didn't impact Liverpool's performance at all. While this was helped by a woeful Watford showing, the Reds manager also deserves credit for his side's supreme performance. He was even able to rest a few players ahead of the Champions League next week.
Player ratings (1-10; 10 = best, players introduced after 70 minutes get no rating)
DF Trent Alexander-Arnold, 8 -- Played some superb passes forward, including one to create a chance for Salah. He also looked confident defensively and made a brilliant recovery tackle on Cucho Hernandez, even though the Colombian would have been offside if it wasn't made. Had a mis-hit shot wide moments into the second half, while another strike was deflected agonisingly past the post.
DF Joel Matip, 6 -- Looked confident throughout the game, whether he was on the ball or on the rare occasion he was being worked defensively.
DF Virgil van Dijk, 7 -- Played a great ball to Salah early on and was there to step in and snuff out any awkward moments at the back when needed.
DF Andrew Robertson, 8 -- Made some brilliant runs forward throughout the game, with one creating a chance for Salah. The Scotland international didn't look like being beaten when he didn't have the ball.
MF James Milner, 7 -- Took up some intelligent positions throughout and did brilliantly to pick out Firmino for the second. Even put in a good shift at right-back after Alexander-Arnold went off.
MF Jordan Henderson, 7 -- Captaining Liverpool for the 150th time, he moved the ball efficiently and was always committed to the press when needed.
MF Naby Keita, 6 - Did well to help out defensively when needed, moved the ball well and saw his deflected shot from range hit the crossbar before going over. Sent a later effort over the crossbar.
FW Mohamed Salah, 10 -- Had the better of left-back Danny Rose from the very first whistle. Provided an outstanding assist for Mane's opener from a pass with the outside of his boot. Then, having been denied various times before, the Egypt international scored a simply exquisite goal for Liverpool's fourth.
FW Roberto Firmino, 10 -- Often linked play well. Took up great positions and showed awareness to pounce on the opportunities that came his way, providing simple finishes for his hat trick.
FW Sadio Mane, 9 -- Made a well-timed run and had a very composed finish for the opener, which was also his 100th Premier League goal. While one backheel almost led to a Watford chance, another for Milner helped him set up Firmino for Liverpool's second. Should have had a second goal himself after Firmino dispossessed Rose and put a chance on the plate for him.
Kostas Tsimikas, 6 -- Replaced Robertson in the 64th minute and almost found Firmino with a cross. Worked well down the left and was unlucky to have a foul called against him after challenging Juraj Kucka on the edge of Watford's box.
Alex Oxlade-Chamberlain, 6 -- Replaced Alexander-Arnold in the 64th minute and looked comfortable, making a few good drives forward.
Neco Williams, N/A -- Replaced Milner in the 83rd minute for his first Liverpool minutes of the season and assisted Firmino for the striker's hat trick.