McClaren has revealed Heskey was a certainty to face Russia on Wednesday as soon as the final whistle went against Israel four days earlier.
The rumours were only fuelled by the sight of McClaren talking to both players individually at England's London Colney training complex on Tuesday morning.
However, McClaren has now confirmed his decision had been made a long time before that.
Having spotted a unique bond between Heskey and star striker Michael Owen, the England coach had no desire to split the partnership.
"I decided Emile would play on Saturday night," said McClaren.
"Last week's training was key for a lot of players. It gives a good insight and I was left in no doubt the partnership of Heskey and Owen would cause problems."
McClaren accepts he has no real idea why Owen should thrive alongside Heskey more than anyone else.
But three goals in two games for the Newcastle forward this week - which saw him become only the fourth player in England history to hit the 40 mark - suggests something special exists between them.
"Sometimes it just happens," said McClaren.
"I don't know why but you just gel and blend with someone."