The Bulldogs coach, who left the Sea Eagles last year one week after winning his second premiership, would've had his heart in his mouth as his former side mounted a stirring comeback after being down 12-0 at halftime at Brookvale.
The match wasn't sealed until man-of-the-match Ben Barba scored his second four-pointer with two minutes remaining, capping off a sensational performance that also included two try-saving tackles.
The Sea Eagles had been waiting for Hasler's return all season and started accordingly.
In the 16,080-strong crowd, anti-Hasler banners waved in the breeze while the two-time premiership-winning coach was booed as his picture flashed up before kick-off.
Co-captain Jason King levelled imposing Bulldogs prop Sam Kasiano with a huge hit in the third minute, made even more physical with a brutal head clash that left Kasiano with a suspected cheekbone fracture.
Manly peppered the Bulldogs tryline early, but it was the visitors who came up with the points, typically through prolific try-scorer Barba.
The brilliant fullback chased his own grubber after 19 minutes and was in the right spot to accept a wayward offload from his opposite Dean Whare to dot down under the posts.
He would do the same for the last try of the match, when he caught his own grubber as it rebounded off the goalposts to seal the match.
Canterbury's first-half blitz also included a perfectly-executed play from five-eighth Josh Reynolds, who kicked on the first tackle from a scrum into the waiting arms of winger Sam Perrett.
It was a particularly courageous performance by the high-flying Bulldogs, who lost Kasiano and halfback Kris Keating, who suffered a hamstring injury, after just 21 minutes.
Bulldogs centre Josh Morris looked to have scored the match-sealer in the 72nd minute but was denied when video referee Steve Clark found a knock-on from teammate Dale Finucane in the lead-up.
After the Bulldogs claimed first points of the second half with a penalty, the Sea Eagles kick-started a comeback with tries to their co-captains coming in a 10-minute period.
First, centre Jamie Lyon snuck around Barba, who was trying to shadow a Daly Cherry-Evans kick over the deadball line, to dot down.
It was Barba's only blemish of the night, but it cost the Bulldogs six points and then prop Jason King accepted a short pass from Anthony Watmough to crash over next to the uprights to reduce the margin to just two.