The Hornets claimed a deserved victory at Vicarage Road to halt a remarkable unbeaten run by the Reds that had stretched over 44 Premier League matches in 14 months.
Ismaila Sarr’s second-half double was followed by a Troy Deeney strike as the home side capitalised on a Liverpool performance that fell short of the form that has seen them take a 22-point lead at the summit.
“Watford were at it from the start and we took time to get going. We had a couple of good moments but nowhere near enough for this Liverpool team,” Robertson told Liverpoolfc.com post-match.
“It’s a performance that has not been expected for a number of years by the fans and we can only apologise for this performance. It’s up to us to bounce back.
“Up to this point it has still been a fantastic season but today we weren’t at the races, we weren’t the Liverpool that everyone has watched.
“It’s up to us to now get back to that because that’s the only way we’re going to win games and get closer to our targets.”
Liverpool were restricted to one shot on target on the evening, with substitute Adam Lallana’s 20-yard strike off the post the closest they came to netting.
Robertson continued: “It’s up to us to now bounce back because today wasn’t good enough, simple as that. Watford deserved the three points, they were brilliant to a man. And we weren’t.
“It’s up to us to now show that this defeat hurts and up to us to now go and put a run together that will crown us champions.”
Despite the disappointing defeat, the Reds remain in control of their own destiny in the final 10 fixtures of the Premier League campaign.
Twelve points is their target to guarantee top spot, while a trip to Chelsea in the FA Cup fifth round on Tuesday represents their first opportunity to put things right.
“Our message has always been the same: we can’t get carried away,” said Robertson.
“Of course, fans are going to get carried away and enjoy the moment we’re in and that’s nice for us because if our fans are enjoying the way we’re playing and what we’re doing, it means we’re doing something right.
“For us, we know how hard this league is. Today we were hoping to make it only three wins away but we’re still four wins away and it’s going to be tough but we believe we can do it.
“That’s up to us to now show that and bounce back against Chelsea in the cup, then we’ve got another massive game against Bournemouth.
“We really need to put in a proper LFC performance and show everyone that this was just a blip, that now we’re going to crack on and get to where we want to be. Everyone knows where that is – but it’s up to us to now show it.”