Alexander-Arnold whipped the free-kick into the top corner from 25 yards out, completing a stunning strike to give Liverpool vital breathing space and make Watford heads drop, as the Reds went on to win 3-0.
"It's a nice moment, I really think this group of players deserves it". We never controlled games in the manner we do now.
Klopp said: "It's nice, the 33 points are brilliant and the goal difference is really good, I like all that".
The Reds found the opening hour hard against Javi Gracia's side, however goals from Mohamed Salah, Trent Alexander-Arnold and Roberto Firmino in the final 22 minutes saw them claim a "mature" victory. On the other side, we had to stay stubborn as well, we had to pass, to be patient but stay lively and accelerate.
Liverpool are next in action on Wednesday night, away at Paris Saint-Germain in the Champions League.
The result was slightly marred after skipper Jordan Henderson was sent off for two tactical fouls in the second-half.
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"It was exactly how it had to be in our situation. We have to win football games". I'm not sure we had the ball in the past as much as we had here as they have been much more open games. I like that. I know a few people miss one or two situations which we had in the last season, especially the second part of the season. OK, we know that.
With the score level Robertson clipped Will Hughes in the box but referee Jon Moss ignored the hosts' protests.
Klopp appeared to be ready to replace Henderson with Fabinho before the red card and he conceded that being without the influential 28-year-old for next weekend's game at home to rivals Everton is a blow.
Liverpool travelling to Watford lingers loathly in the memory as potential banana skin and the first half made those lingering doubts seem well-founded.
"Coming back after an global break and playing a team who only had three or four players away I think, meaning they could train properly - we could only train Wednesday and Thursday - and to play against such an organised team is hard".