Liverpool came from behind twice in an indifferent start to the Premier League season but conceded in the 94th minute to share the points.
Without Coutinho and his 'back' injury, and with Lallana a medium term injury - the reds opted for a front three of Mane, Salah and Firmino.
But Watford were more than Liverpool's match - bullying the reds and exposing Liverpool's long-standing defensive frailties that have not been improved upon in the past three months.
There were only 8 minutes on the clock when Watford sent in a corner that Okaka was allowed a free header to put it past Mignolet. Marking? Non existent.
Liverpool were toiling - the midfield looked weak but just before the half hour Sadio Mane put Liverpool level with a sublime goal that arrived from nowhere.
But the reds were behind again within two minutes - Doucoure restoring the advantage for Watford and they held their 2-1 lead into half time.
Salah and Firmino had been quiet in the first half - principally due to a lack of support from midfield but in the second half they would have more say.
In the 55th minute Emre Can started an LFC move on the half-way line that saw Salah's explosive pace inside the box result in an overstretched defensive tackle - and a penalty awarded to the reds.
Firmino stepped up and made it 2-2 before running straight to collect the ball.
That urgency powered through the team and two minutes later the reds were in front for the first time in the game.
Lovren lifting the ball forward, Firmino drawing the keeper out towards him before looping the ball over for Salah to enjoy the simplest of tap ins.
A goal on his debut for Mo Salah - joining the likes of Suarez, Sturridge, Fowler, Owen, Collymore, Wark and even Billy Liddell as LFC debut goalscorers.
There was a sting in the tail to come as from another corner Richarlison bundled in his shot in the 94th minute to give Watford a deserved spoil of the points.
This wasn't a pretty performance from the reds; and on reflection Liverpool still appear to have the same defensive and goalkeeping flaws that they simply haven't addressed in the past 10 weeks in the transfer market.
But Salah has started as well as last season's principal signing Mane and alongside the negatives from this game there are some positives.
Next up for the reds - a trip to Germany for the first leg of the Champions League play off. Whether Coutinho's dummy is back in the pram or not remains to be seen.
