The club's head of football also vowed to make any improvements necessary across the code to ensure such a tragic event was never repeated.
Keith Titmuss suffered a seizure after a cardio workout at the club's base at Narrabeen, on Sydney's northern beaches, on November 23, 2020.
He died five hours later in hospital.
After giving evidence, Hasler addressed Titmuss's father Paul and mother Lafo (both pictured). (Bianca De Marchi/AAP PHOTOS)
After giving evidence at an inquest into his death, Hasler addressed Titmuss's family, including father Paul and mother Lafo, directly in the NSW coroners court.
"Keith was a beautiful boy, he was much loved and very highly regarded and respected," the premiership-winning coach said on Friday.
"I am so, so sorry and I share your heartbreak, I really do."
Manly head of football John Bonasera became emotional while giving evidence and when given an opportunity to address Titmuss's family.
"I know nothing could be said here that would ever ease your pain ... but know at the time we did everything humanly possible," he said.
"It would be (Keith's) legacy that nothing like this would ever happen again."
An inquest into the 20-year-old's death previously heard he most likely suffered exertional heat stroke, although an autopsy was unable to confirm the exact cause.
The inquest previously heard Keith Titmuss most likely suffered exertional heat stroke. (HANDOUT/MANLY WARRINGAH SEA EAGLES)
Hasler, now head coach of the Gold Coast Titans after his 2022 sacking from the Sea Eagles, said young players in particular were often eager to prove themselves.
But it was up to the club to create a culture that made it acceptable not to overexert themselves and cause an injury.
"From the outset, particularly for younger players, there is a need not to overstride ... to overcome that emotion of, 'I must succeed, I must impress the coach'," he said.
"It's essential (for them) to understand the last thing we want to do is to injure them or not to have them on the field, that's really important as part of our culture."
Hasler said there were no "red flags" raised with him. (Bianca De Marchi/AAP PHOTOS)
Hasler said the session would have been challenging as it was the first following the off-season break, but there were rest breaks so that players could recover and rehydrate in between cardio phases.
He added there were no "red flags" raised with him about Titmuss's physical capabilities after the off-season.
The inquest previously heard one player describe the session as a "nine out of 10" for intensity.
But Hasler said he would rate the intensity as between six and seven out of 10.
"It was the first training session back with the new group so we were very cautious of not overloading," he said.
"This is new to them, it makes no sense to get a player to go beyond what they can because we want to keep them on the field, keep them training and get them to the stage where they can compete."
Mr Bonasera said the club had implemented measures to reduce the risks of heat illness following Titmuss's death, including installing air conditioning and cold pools at indoor training areas.
The club has also taken steps to educate players about the risks, signs and symptoms of heat stroke.
"Making sure everyone has the appropriate level of knowledge, and recognition within the players themselves, is key," he said.
"We need to make sure that we do foster an environment where the players can speak up ... and it's sad that we needed to go through an incident like this to be the catalyst."
The inquest continues on Tuesday.
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