Only this time, the negativity wasn’t all his doing.
Call it a drab 90 minutes and the need to create talking points, but the focus in the aftermath of Costa’s scuffle with Hornets defender Juan Carlos Paredes seems somewhat contrived.
In the post-match press conference, Guus Hiddink was quizzed on four separate occasions regarding Costa’s antics after he and Parades had partaken in a game of handbags late in the first half.
"It was very clear because it was very close. It was just 10 yards from our bench and Paredes punched Diego in the back," said Hiddink.
"After that they stumbled, and then Parades brought his hands to his head. Diego was nowhere near his head—they stumbled.
"This is something that we must not do, which is a provocation to get someone sent off the pitch. Happily the referee and the assistant referee saw it, so they didn't go with this provocation.
"It’s a pity that it happened. There was no way that Diego was close to his face."
That the story was being probed and run with to that extent was even more of a pity. Costa has rightly been criticised in the media for his behaviour at times since arriving in English football in 2014, yet this time he deserved a little more regard.
It was a sign that Paredes' provocation of Costa that Hiddink described seems to have extended to the media. Anything he does is put under the microscope and dissected to the nth degree.
That simply isn't right.
English football thrives on the controversy of the bad guy, and right now nobody fits the bill more than Costa. He's the Wild West outlaw being hunted by a posse, only this one includes characters from both sides of the law.
The Chelsea striker has more than played his part in things reaching this point. The Spain international has pushed the boundaries much too often; he’s seen the line and wilfully stepped over it.
He didn't against Watford, but that fact wasn't recognised. When Paredes went down clutching his head, the headlines were already being written—Costa was at it again.
Here's the point, though. If we’re to frown upon players who deliberately fool referees with simulation in the box to win penalties—a charge of which Chelsea have been guilty of themselves in the past—then the provoking of players, or in the case of Parades, writhing around in phantom pain in the hope of picking up a numerical advantage, must be equally condemned.
It shouldn't matter who the player is or which club he plays for.
The issue at Vicarage Road is that it wasn’t. For whatever reason—whether it was Paredes' profile or lack thereof—Costa became the instant focus, with Paredes mentioned merely because he was the Chelsea man’s dance partner.
The irony here is Costa has previous when it comes to provocation—think Gabriel Paulista and Arsenal in September.
His past tells us he can hardly complain.
Unlike Paredes, though, Costa was taken to task over that incident, and it led to a three-match retrospective ban for his part in Gabriel's red card. The Arsenal man's ban was also rescinded by the FA.
"Of course [it disappoints me]," Hiddink continued. "This is a man's game, and our central defender, their central defender and other club's central defenders are big guys.
"Sometimes they go over the edge, and that's what we like when it's a man's game, but it must not be going to where there are provocations."
Where Hiddink should be disappointed is that Costa allowed those incidents off the ball to affect his overall performance. Before Paredes thought he may have struck gold, Costa was already getting tangled with Craig Cathcart and Sebastian Prodl.
He had the look of the Costa we saw all too often this season before Jose Mourinho's departure in December. He was playing on the edge, but it was being motivated by the wrong source.
After the interval, Costa got back to being the player who has been on fire post-Christmas. He was back playing through the middle, making runs off the ball to give Cesc Fabregas a moving target to pick out.
Costa nearly won the game for Chelsea with a header that was expertly kept out by Heurelho Gomes late on. Had he played like that from the off, Hiddink maybe wouldn't be lamenting more dropped points in Chelsea's attempts to climb the Premier League table.
In the second half, it was the Costa we wanted to see; it's the player we should have been writing about.
Two home fans sat next to the press seats at Vicarage Road were incandescent that Costa wasn't sent off for his part in the Paredes incident. Unlike most of the 20,000 or so people inside the stadium, they were able to view the TV replays available to the media—the TV replays that showed Costa to be more the victim of a stitch-up if anything.
"Mike Dean should've still sent him off just for being a c--t," one responded as he watched over the shoulder of a journalist.
The concern is that sentiment toward Costa is beginning to run a little deeper than just those two.