The surprise loss ended the Reds 44-game unbeaten run in the league.
Watford also became the first team to beat Liverpool in the league this season.
It was goals from Ismaila Sarr (2) and Hornets captain Troy Deeney (1) that nailed the EPL leaders.
Reds captain, Van Dijk told post match press briefing, “Credit to Watford, they deserved it.
“The chances they created, they stuck to their plan and scored three goals.
“We didn’t create too many big opportunities, but we shouldn’t take the credit away from Watford.
“We have to look at ourselves and improve again.
“It’s a tough one to take but that is football. Today should have been better, but whole season we have been good defensively.
“We know there is room for improvement, and we will improve, that’s the good thing about our mentality.
”It is not acceptable to lose here 3-0 and in this way as well.”
“Losing hurts but the records are only for the media, we didn’t mention it once.
“We want to strike back and show what we’ve been doing the whole season,” he added.
