The Blues boss had named the same starting line-up for the last eight Premier League fixtures but opted to make five changes at the City Ground with Stuart Parnaby in for the suspended Stephen Carr, while Keith Fahey, Damien Johnson, Gregory Vignal and Kevin Phillips also got rare starts.
Asked about the changes and whether it was an indication of his priorities lying elsewhere, McLeish, whose side sit eighth in the Premier League after an 11-match unbeaten run, said: "Not really because they were enforced.
"Carr was suspended, (James) McFadden was a bit tight on his hamstring and Scott Dann had a bit of tenderness in his knee.
"Maybe it could be seen as a calculated risk to keep the momentum going if it was a Premier League game but I feel we have enough experience and ability in reserve to come in and they slotted in very well.
"I understand they are perhaps lacking a bit of match practice but that didn't stop those players coming in and giving us everything.
Kevin (Phillips) came in and looked dangerous and could have had a couple of goals.
"I would have liked to have won it, but Forest had a lot to say about that.
"They had their chances and we also had our chances, but we'll take the draw. We're still in the cup."