Benitez has signed initially on loan in a deal which could eventually be worth around £7million from Mexican club Santos Laguna.
His pre-season work was put on hold because of the protracted nature of the deal and while he recovered from a shoulder operation.
But the Ecuador international impressed as a substitute during the second half of Sunday's single goal defeat by Manchester United at Old Trafford and came close to grabbing a shock equaliser.
McLeish said: "Will we start with Benitez? We are tempted but we have to be sure he is ready. He is close to it.
"We will mull it over in the next 24 hours but he is still an unknown quantity in this country.
"We have seen him in action in the South American Championship, playing for his club team and Ecuador, but now he has got the big challenge of doing it in the toughest league in the world.
"He is small, he can turn defenders. We haven't seen a lot of him football-wise in the last four or five months.
"He has only played 45 minutes in a behind-closed-doors match last week and 20 minutes at Old Trafford. He is very short of match fitness.
"But he has done the training programme so his pre-season fitness should be there. It is about the match sharpness now and the fans got a wee glimpse of him at the weekend and were quite excited by what they saw."
McLeish is still hoping to complete the signing of Sporting Gijon midfielder Michel in a £3million deal.
He said: "We have opened talks and we will see if we can do it. The 'i's' are not dotted yet and the 't's' are not crossed and I never like to talk about a deal until it has been done.
"He is a good athlete, has terrific technique and touch as you would expect from a Spaniard and he brings a bit of youth to the side and enhances the quality of the squad."
McLeish will continue to try and strengthen before the end of the transfer window and is aware home form will be vital to Blues' chances of survival.
But he played down the claims that Pompey will be pushovers after losing a series of high profile players this summer.
He said: "When I look at the players in their team at the weekend, I saw nothing but household names, guys who have been in the Premier League a long time.
"I have massive respect for Portsmouth. We are the rookies in this encounter. The big advantage is we are playing at home and in front of our crowd we don't fear anyone."
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