Symonds is considering his future at the top level after being sent home from the current one-day series against Bangladesh for going fishing last Friday rather than attending a compulsory team meeting.
The Australians face an extremely testing tour of India in October as they attempt to defend the Border-Gavaskar Trophy they won in a controversial series earlier this year. In that series lingering on-field angst between Symonds and Indian spinner Harbhajan Singh dominated the headlines.
Nielsen, who had previously admitted concerns over Symonds' state of mind, told Australian Associated Press on Monday that there was no way to guarantee he would be available for the India series.
"I don't think we can at the moment, I don't think we can bank on much at all at the moment," said Nielsen.
"The only thing we can bank on is providing as much support as we possibly can for him to make the right decision for him and for us, to come back into the group 100 per cent committed. But how long is a piece of string?
"I don't know if Roy (Symonds' nickname) would understand or know how long it's going to take. I certainly don't."
The Australian squad for the series is expected to be named within weeks and Nielsen admitted it would create a significant void if Symonds was not available. But he said there was no timeline for a decision from the player.
"He could decide that he needs a bit more time than we thought," Nielsen said. "We just don't know, that's the hard part."
Symonds said on Monday he wanted time to consider his future.