The Manor Racing rookie, the first Indonesian on the starting grid, has been following his faith in Montreal by not eating or drinking from dawn until sunset during the holy month that started on June 6.
His manager Piers Hunnisett recognised there were concerns that the lack of food and fluids could affect the 23-year-old's concentration.
"Rio has been fasting this week but the team and his physio will decide what to do on Sunday," he told Reuters on Saturday. "It's a long race and there is a worry about dehydration.
"Drivers lose a lot of body fluid during a race and we don't want him losing concentration when he is driving at 350kph or whatever it may be.
"It's more of a safety issue. He has been following his faith in Montreal this week but we will see what the advice of the team doctors is. But on Monday he will be fasting again."
Hunnisett said talks were continuing with the Indonesian government and other sponsors over extending Haryanto's deal with the British-based team.
Haryanto’s primary sponsor is state-owned oil company Pertamina, which so far has contributed five million euros ($5.62 million) towards the funding needed to secure the race seat at the sport’s smallest team.
The Indonesian has a guaranteed drive until the 11th round of the season in Hungary but needs more money to remain in the seat beyond the summer break.
"The contract talks are still ongoing," added Hunnisett. "Rio is in the car until Hungary, it's all paid up to date, it's just whether we extend it.
"The news in Indonesia is huge and the government love it. He is doing a good job and hopefully we are here for the long term. He has surprised a lot of people," he said.
"His team mate Pascal Wehrlein was tipped to go places and if we can match him it's very good for Rio's career. The team like him, the engineers like him but it comes down to pounds and pence in the end."
Haryanto will start Sunday's race from 20th place on the grid after hitting the barriers in the first phase of qualifying.
($1 = 0.8890 euros)
(Editing by Alan Baldwin)
