The 40-year-old, who won gold at London 2012, was not on the list of 43 names announced by Team GB on Thursday, after she failed to meet testing times in the women's eight.
But she is expected to be selected for the double scull with Vicky Thornley.
"We've got to help them make sure it happens," said coach Paul Thompson.
"Katherine is very single minded. She is definitely looking forward and she hasn't stepped away from her ambition at all."
Should she win a medal at her fifth Games in Rio, Grainger would become Britain's most decorated female Olympian, having also won silver medals in Sydney, Athens and Beijing.
After her name was excluded from the initial Rio line-up, British Rowing performance director Sir David Tanner explained the situation.
"We did some testing at the beginning of this week to see whether or not the double of Katherine Grainger and Vicky Thornley could, at their request, make a slot for themselves in the women's eight. That was not to be," he said.
"It's my confident intention very soon to be announcing those two back in their double and racing for Team GB at the Rio Games.
"There is no uncertainty. The only issue is that we weren't ready to get the announcement in in time for today. It's quite straightforward."
Britain's Katherine Grainger remains focused on a fifth Olympic medal at Rio 2016, despite missing out on initial selection.