A day on from manager Jurgen Klopp signing a contract extension to 2024, the runaway leader was well below its brilliant best but nevertheless claimed victory through two goals from Mohamed Salah, the second coming in the 90th minute.
Abdoulaye Doucoure and Ismaila Sarr were guilty of horrible misses for Watford either side of Salah's first strike seven minutes before half-time and there were several other presentable chances for Watford in its first game under Nigel Pearson.
However, Liverpool, which also had a goal ruled out by VAR (video assistant referee), ultimately did enough to claim victory and will now hope for a positive update on Wijnaldum as it prepares for a FIFA Club World Cup semi-final on Thursday (AEDT).
Liverpool's previous three home games against Watford had yielded an aggregate score of 16-1, but this was certainly no cakewalk.
Despite having less than 30 per cent possession in the first period, Watford regularly threatened on the counter-attack, enjoying success down the flanks.
However, both Doucoure and Sarr mis-kicked horribly when given clear sights of goal in the Liverpool box and the first of those misses was punished inside 58 seconds as the hosts scored from their only shot on target prior to half-time.
An incisive Liverpool break ended with Salah latching on to Sadio Mane's pass into the left channel, before cutting inside Kiko Femenia all too easily and curling a gorgeous finish beyond Ben Foster into the top right-hand corner.
Watford continued to look dangerous after the break and Alisson was forced into stops from Sarr and Gerard Deulofeu either side of Mane seeing a 50th-minute goal ruled out for offside following a VAR review.
A lack of quality in the final third was prevalent thereafter, with Roberto Firmino particularly sloppy, but Liverpool, which is now 11 points clear at the top of the table ahead of Leicester City's clash with Norwich City, eventually added a second as Salah flicked home a scuffed shot from substitute Divock Origi.