Villa received around ?36million after selling Young and Downing to Manchester United and Liverpool respectively with N'Zogbia brought in from Wigan.
He has struggled to make a telling impact so far but impressed as a substitute in the midweek Carling Cup defeat against Bolton.
McLeish, whose side visit QPR on Sunday, said: "Let's not kid ourselves, losing guys like Ashley Young and Stewart Downing can hurt anybody.
"We lost a couple of really good players in those two amongst others who left at the end of the season (Nigel Reo-Coker, Brad Friedel).
"But we've got players in and it's now a challenge for them to show that they can emulate these players."
McLeish came close to signing N'Zogbia when manager of Birmingham and is confident he will eventually show his true form.
He said: "Charles is a player who, for me, in the last two months of last season did a lot to keep Wigan in the Barclays Premier League.
"There's been some decent play from him with us. I thought he did well on his debut and against Wolves.
"But the final ball that we're used to seeing him delivering has just eluded him at the moment.
"He'll get that. He's looking much, much brighter, he came on the other day (against Bolton) and did well.
"He believes in himself. We've just got to keep believing in him."
McLeish is looking for a positive reaction after the Carling Cup exit after speaking with his experienced players at the end of the game.
He said: "We need a reaction. I think on the night Bolton were a bit more experienced and streetwise than us.
"We definitely got into good positions, but the final ball or the retention of the ball wasn't up to standard. So we have to get those standards raised.
"I wanted to hear a couple of the experienced players' points of view after the game and it was fine. It was a good debate.
"We're not the only Premier League club to get knocked out of the Carling Cup. But we don't like losing at home and the fans don't like that either."
Villa's record signing Darren Bent is ruled out of the trip to Loftus Road with his groin problem but will not require surgery.
But his involvement with England for their Euro 2012 qualifier with Montenegro on October 7 will depend on him being fit to face Wigan next weekend.
McLeish said: "He's not bad. We think he'll be ready for the next game.
"There was a bit of fluid but I think the scan was quite good in terms of good news for Darren.
"There wasn't anything that would suggest it would keep him out longer or he would have to get any kind of operation.
"Will he be available for England? That depends on him being fit for us next weekend."
Emile Heskey (hamstring) is still unavailable and on-loan Tottenham midfielder Jermaine Jenas is given a 50-50 chance of recovering from a thigh problem.
Source: Team Talk
Source: Team Talk