Keane is in the Villa squad to face his former club Wolves at Molineux in Saturday's derby.
But McLeish will include the Los Angeles Galaxy striker in his first 11 when he feels the time is right.
Keane has been building up his fitness and sharpness after taking a three-week break at the end of the Major Soccer League season and appeared as a substitute against Everton last weekend.
He is with Villa until the end of February, and McLeish said: "There isn't any pressure to start Robbie. He is not here for long but it has to be the right time.
"We are just assessing him fitness-wise. He will be very eager to play in this one (against Wolves) but he is the kind of lad who wants to be involved whoever the opposition is.
"We've got another couple of days' training before the weekend.
"Robbie has been looking sharp so I've got food for thought for the weekend but I don't want to give anything away in advance of the game."
Meanwhile, McLeish has still to decide on the long-term future of midfielder Jean Makoun - currently on loan at Greek side Olympiacos.
Makoun joined Villa 12 months ago but failed to make an impact under the then manager Gerard Houllier.
McLeish said: "Has Jean got a future with us? It depends. The reason for letting him out on loan was that I was looking for a different sort of midfielder."
