Chelsea struggled for intensity until taking the lead through Ruben Loftus-Cheek early in the second half, perhaps because of Thursday’s Europa League semi-final first leg at Eintracht Frankfurt, and Sarri said: “We were in trouble because we were tired, physically and mentally.

“Then, in the second half, (Watford) lowered their intensity so we were in control of the match. We were lucky, of course, because we scored after two minutes. After the first goal, probably we improved mentally and we were able to play very well for 30-35 minutes.

“In the first half we were tired and in trouble, tired more mentally than physically. But it’s not easy to play (again) after 60 hours.

“We were not able to play simply. In the second half we did it better, better and better, probably because something changed in our minds after the first goal.”

One concern for Sarri is N’Golo Kante’s fitness. The energetic midfielder was substituted and replaced by Loftus-Cheek after suffering a hamstring injury, making him a doubt for a crucial week in which they host Frankfurt in the second leg of their last-four clash before visiting the Foxes.

Discussing Kante’s condition, Sarri said: “The problem is with his hamstring. But I made a mistake because, probably, he had to rest. Sometimes it’s very difficult to put on the bench a player so important like (Eden) Hazard, like Kante, but you have to do it.

“We have to wait until the morning (to determine the severity of the injury), but it’s very difficult to recover him for the next two matches. We have to try and recover him for the (potential Europa League) final.”

After Loftus-Cheek’s header, a further header from David Luiz and then Gonzalo Higuain’s close-range finish increased the hosts’ lead, leaving Watford 10th.

Their manager Javi Gracia recognised they may have been distracted by the prospect of their FA Cup final against Manchester City later this month, saying: “We have to play the final. It’s normal for it to be on our minds, but we were focused on this game (against Chelsea).

“I like the way we played the first half but, in that moment, you have to take advantage of the chances you have. If you don’t do that against teams like this, they will have periods where they create chances, and they did that.

“We have to finish the league next week, and then have an important final. If your behaviour always is defending badly at corners and free-kicks, I’d be worried. But it’s not like that. I’d prefer to be optimistic.”

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