Keith Titmuss, 20, suffered a seizure after a gruelling indoor workout session at the Sydney club's northern beaches base on November 23, 2020.
Titmuss was transported to Royal North Shore Hospital but died five hours later having suffered a cardiac arrest.
Deputy State Coroner Derek Lee found the budding NRL player's cause of death was exertion heat stroke, with a pre-existing heart condition unlikely to have played a part.
Delivering his findings on Friday, Mr Lee urged the NRL to look at mandating a two-week acclimatisation period when players return from off-season and use the death as a case study for awareness on heat stroke.
Mr Lee also recommended the game's governing body, as part of a heat policy review, consider mandating that clubs report instances of heat stroke.
He urged Manly to review its record-keeping processes on adverse health events like heat exhaustion to make sure they were "robust and reliable".
The pre-season workout at the club's Narrabeen gym, which followed an outdoor session, was "more likely than not inappropriate", the coroner found.
Mr Lee said when Titmuss entered the "very hot and very humid" dojo he was likely suffering dehydration before developing exertional heat stroke.
Citing testimony, he said the demanding workout was aimed at pushing players past fatigue, while noting Titmuss, although very fit, had a low level of aerobic fitness among the playing group.
It was unclear if a fan in the gym was operating at the time, and ice nearby could have been applied to him although it was unknown if that would have helped alleviate Titmuss' symptoms, he said.
An attempt to treat Titmuss for hyperventilation by using a paper bag was "misguided", the coroner found.
Evidence at the inquest was that paramedics at the scene noted Titmuss' temperature at 41.9C, while his heart rate was over 140 beats per minute.
A former teammate told police the session ranked a nine out of 10 on intensity, but Manly coach at the time Des Hasler rated it a six or seven, the inquest heard.
Mr Lee said Titmuss' death was "tragic and devastating", describing the young man as kind, gentle, respected and "salt of the earth".
"His friends called him the pied piper of his group," the coroner said.
Outside court, his mother Lafo Titmuss said she was pleased to get certainty on her son's cause of death and urged all involved to adopt the recommendations.
Flanked by family and supporters, she said she wanted the NRL to emphasise the importance of heat policy and not underestimate the impact of heat.
Ms Titmuss has previously said she hopes recommendations from the inquest will be instructive for the NRL and other sporting codes.
Manly chief executive Tony Mestrov said a death like that of Titmuss can't happen again in the game.
"We all understand that and we don't want it to happen under Manly's watch ever again," Mr Mestov told reporters.
Comment has been sought from the NRL.
