Hendrick, the club’s £10.5m (€12.1m) record signing in the summer, nodded home Steven Defour’s 38th-minute corner and the Belgian bagged a second assist when Keane rose to head a second shortly after half-time.
Defour has now scored one and set up three of Burnley’s five goals this season, and here provided the kind of spark that was visibly lacking from the visitors, who suffered a sharp comedown having turned over Manchester United last time out.
For the Clarets it was a welcome second win of the campaign, following their surprise defeat of Liverpool.
“We’ve never lacked grit and determination and had the base of a performance. The lads were terrific tonight,” Clarets boss Sean Dyche said.
“I like to play 4-4-2 and attack teams but it was the right side tonight to get a performance. Whatever shape we play we need energy and pressing and tonight there were moments of quality. If you keep clean sheets you’ll have a chance in games.
“Last time around in the Premier League we didn’t concede many so we believe there’s good organisation in the team and we need to be productive at the top end of the pitch.”
The Clarets had all the territory and all the meaningful possession in the first half but, until Hendrick intervened, few clear chances.
Hendrick and Johann Berg Gudmundsson were the likeliest threats, the Irishman always lurking in the space behind Sam Vokes while the Icelander served notice with a couple of tempting crosses.
Half-time was beckoning when Dean Marney intercepted a stray Gudmundsson shot and nicked a corner, allowing Defour to take aim from the right.
The Belgian aimed for Hendrick, who outmuscled Holebas to force his header over the line.
Watford seemed to think Defour’s cross had drifted out on its way into the box but there was no reprieve.
The only recriminations about Burnley’s second, five minutes after the restart, were aimed at defenders rather than officials.
After another Defour corner had come back to him the midfielder crossed again, with Watford this time allowing Keane to climb above them and direct the ball home.
Heaton, Lowton, Keane, Mee, Ward, Gudmundsson (Kightly 90), Hendrick, Marney, Defour (Arfield 83), Boyd, Vokes.
Gomes, Amrabat (Success 58), Cathcart (Zuniga 46), Prodl, Britos, Holebas, Pereyra (Kenedy 75), Behrami, Capoue, Ighalo, Deeney.