Jackett made 428 appearances as part of the successful Hornets side of the 1980s, but his first meeting against his old team as a manager ended in total humiliation on Saturday.
The Lions kicked off in fourth place in the Championship and on the back of a 21-game unbeaten run at The Den, but they were blown away by first-half goals from John Eustace, Jordon Mutch and Marvin Sordell.
Adrian Mariappa then made it four after the break, and Liam Trotter pulled one back before Danny Graham and Martin Taylor completed the rout as Watford made it back-to-back away wins to leapfrog the south Londoners in the table.
Watford had the hosts on the back foot from the first whistle and were rewarded when they went ahead in the seventh minute.
Don Cowie swung in a corner which flew across a crowded penalty area and found Eustace, who diverted the ball into the net with his thigh at the far post.
The Lions defence looked shell-shocked and Watford took full advantage, doubling their lead with 13 minutes on the clock.
Birmingham loanee Mutch saw his initial shot blocked but was allowed to have another crack and lashed the ball past David Forde for his first goal for the Hornets.
Steve Morison, with five goals to his name so far this season, was the man most likely to haul Millwall back into the game.
The Wales international came agonisingly close to halving the deficit when his angled half-volley crashed against the underside of the crossbar and bounced clear.
Lions skipper Paul Robinson then headed James Henry’s corner goalwards but Watford’s England Under-21 international Scott Loach made a fine save.
And Watford got a third on the stroke of half-time when Sordell grabbed his fourth goal in three games, prodding away the rebound after his first shot was blocked with Millwall’s defence dithering again.
The Den faithful were expecting a second-half onslaught from their side, but instead they found themselves four down after 54 minutes.
Cowie sent in another corner and centre-half Mariappa powered his header past Forde.
Millwall finally got on the scoresheet two minutes later, Trotter finishing well after latching onto Scott Barron’s cross.
But a comeback did not materialise and the tireless Graham grabbed a fifth for Watford when he rifled his shot past Forde before Taylor headed the sixth a minute from time.