Surely there's a tipping point somewhere? I don't necessarily say it would be after Watford if we lost to them but every loss makes it more likely than the last.
There's a cumulative effect and 4 out of 5 defeats in the league is worse than 3 out of 4 just as 5 out of 6 would be worse still.
Currently we're 5 points outside the top 4, we were 10 behind when Mourinho was axed. A defeat to Watford could make it 8. Certainly pushing the same territory especially when it's Chelsea after that.
As for it being Watford specifically as a team, who knows, but if the aim is always top 4, and it looks like it often has been when it comes to managerial sackings then losing to teams we're not striving to finish ahead of in the minds of the Glazers might not be so bad, however humiliating the defeat. Losing to those at the other end of the table could well be seen as worse.
Not saying all this is the case, but it's certainly possible that defeat to Watford could be a tipping point. Of course it's all guesswork, I wouldn't be saying it surely would like the other poster, but then you seem overly confident that it wouldn't be. Nobody really knows.
Could even be that Watford isn't a final tipping point but that a bad result there means we have to beat Chelsea for him to save his job. On the other hand, beat Watford and perhaps a draw against Chelsea would be seen as enough.
Again it's guesswork, but the Watford game/result is part of his cumulative body of work so could a tipping point as much as any other match.