The Blues have struggled to find consistent form in the top flight in recent weeks and the west London side slumped to a disappointing 2-1 loss to Crystal Palace last weekend.
Chelsea also lost to Manchester City before the international to leave them nine points behind the leaders after just eight games of the new campaign.
Antonio Conte’s men played out a thrilling 3-3 draw with Roma in the Champions League on Wednesday night but they will be desperate to return to winning ways against Watford this weekend.
And former Arsenal star Merson is tipping Chelsea to claim all three points against Watford this weekend.
Speaking to Sky Sports, Merson said: “This is a massive game. Looking at the start of the season, it looked like a gimme for Chelsea. But not now, especially the way Chelsea are.
“They got ripped apart by Man City, beaten by Palace who couldn’t score goals for toffee, and then let in three against nowhere near the best side in Italy at the moment, Roma.
“It’s worrying for Chelsea, and it’s a big game. If they lose this, and Man City win as expected, they go 12 points behind them. People say there’s a long way to go, but City are putting severe pressure on everybody. You look at them and think: where are they going to lose three or four games?
“Watford have done really well, but I’m going to go Chelsea 2-1.”
Chelsea are aiming to retain their Premier League title this season, after having won the trophy in Conte’s first campaign in charge last term.
The west London side have won the Premier League title in two of the last three seasons.
The Blues are currently top of their Champions League group after winning two and drawing one of their opening three games.
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