Costa has been a dilemma for successive Chelsea managers who have had to balance his undoubted talent and ability to impact a game with his flawed character and short fuse which has made him a constant target for referees and a season ticket holder for FA disciplinary hearings.
Even Jose Mourinho, who rather enjoys an abrasive attitude, found Costa too hot to handle and the striker’s dramatic loss of form — and apparent drop in motivation — contributed to the Special One’s departure from Stamford Bridge.
Now Costa is returning to the form which made him such an instant hero in west London.
He scored a dramatic late winner to beat West Ham last Monday and repeated the act at Vicarage Road after Michy Batshuayi had equalised Etiene Capoue’s opener.
All this just 48 hours after giving an interview to ESPN in which he complained he was being harshly treated in England, sparking suggestions he may choose to leave.
“Diego must be focused on the game and not take notice of other players, or managers or the press who say bad things about him,” said Conte. “For me, performance is the most important thing. The other things are nothing.
“He’s an important player. He can score a lot of goals and I want him to put himself in the game and not think of other situations. It’s important for him and for me. Otherwise, it’s not good for Chelsea or the fans and also for the English league, I think.”
The challenge for Conte is to keep Costa happy in England and to slowly adjust the psychological flaws that hold him back, perhaps in the same way that Barcelona have successfully recuperated Luis Suarez.
In the meantime, however, he can reflect on a positive start to the season even if Chelsea were not at their best against Watford, who were the more progressive side for the first hour and who deservedly went ahead with a well-taken goal from Capoue. But Conte’s decision to bring on three substitutes inside seven second-half minutes (Victor Moses, Cesc Fabregas and Batshuayi) turned the game.
Batshuayi, who also hit the bar late on, equalised after a long-range Eden Hazard shot was spilled by Watford goalkeeper Heurelho Gomes — before Costa hit the winner following a beautiful pass from Fabregas.
Conte said: “It was important in these two games to win because you can continue to work as confidence rises. We needed to start in the right way. Now we have to continue. But so far I am happy.”
WATFORD: Gomes 6; Cathcart 6, Prodl 7, Britos 7, Beharami 6; Amrabat 8, Guedioura 7 (Vydra 88), Capoue 7 (Doucoure 84), Holebas 7 (Zuniga 89); Deeney 6, Ighalo 7.
CHELSEA: Courtois 6; Ivanovic 6, Cahill 6, Terry 6, Azpilicueta 5; Matic 5 (Fabregas 73; 7), Kante 7; Pedro 5 (Moses 71; 7), Oscar 5 (Batshuayi 73; 7), Hazard 7; Costa 7.
Referee: Jon Moss