Barton's Newcastle return dependent on FA hearing
There is speculation the FA's regulatory commission could hand the 25 year old a lengthy ban.
Keegan said: "What we have got to do with Joey is just try to assess where his fitness is.
"It is all right to come in and play a game, but how will he react after it?
"I am fully aware he has got his thing on September 5, and it is highly unlikely Joey will be involved with us beforehand.
"If he was fit enough and looked strong enough and put himself up for selection that way, I would play him before then.
"But at the moment, it is looking unlikely just because of the work he has missed."
Barton has not played for his club since the 2-0 home defeat by Chelsea on May 5.