The 26-year-old England international was the subject of a failed £15million bid from the Reds earlier in the week.
There has been speculation Liverpool will up their bid to £20million, but McLeish is determined to hold on to the player after losing Ashley Young and Brad Friedel in the transfer window already.
McLeish admits he has already spoken to Downing about his future and plans to speak to the former Middlesbrough favourite again when the squad returns for pre-season training.
"He had a magnificent season last year and really came to full maturity as a top player," McLeish told the Birmingham Mail.
"I understand why there's teams interested in him and that is what happens when you play well consistently.
"His performances last season were top drawer. That's why he's not for sale. The club don't want to sell him.
"I spoke to him briefly last week and I intend speaking to him again when all the lads are back in for training."
Young left Villa Park for Manchester United last month and the departure of Downing would leave McLeish desperately short of proven options on the wing.