With half an eye on the play-offs if they fail to claim second spot, Hellberg rested four of his regulars, including Scotland forward Conway.
Whittaker’s superb curling free-kick from close to 30 yards out put Boro in front after six minutes, although they were fortunate to retain that lead as Luca Kjerrumgaard spurned two chances to score an equaliser, including one effort that was fingertipped on to the post by Sol Brynn.
Top scorer Whittaker turned provider for the second, although there was a big slice of luck about how Strelec turned it over the line, inadvertently with his head after sliding in.
Watford centre-half Abankwah muscled between two Boro defenders to halve the deficit shortly into the second period, but from there the hosts took control.
Whittaker was most alert when substitute Matt Targett’s low cross ricocheted into his path in the penalty area for his second goal and 14th of the season, and the ex-Plymouth Argyle man came close to a hat-trick with a thumping shot from outside the area that struck the bar.
That was his last act as he was replaced by Conway, and the Scotland forward got on the scoresheet from the penalty spot after Abankwah’s handball.
There was even time for him to add a fifth for Boro, and his fourth in five games, as the home side significantly improved their goal difference.
Watford’s late collapse mirrored their season as, having sat one place behind Boro on New Year’s Day when they were fourth, they have won only three of their subsequent 20 games and plummeted down the table.
