That is because former Killie boss Jefferies completed a surprise return to Tynecastle on Friday night, barely two hours after Csaba Laszlo was sacked as manager.
The change arguably makes MacPherson's job of planning for the game more difficult.
But the Buddies boss insists all his energy is focused on getting off to the sort of start his side enjoyed in going 1-0 up in Saturday's SPL game at Hibernian rather than 1-0 down to Rangers in their previous outing.
"It is difficult to second guess what is going to happen in any game," MacPherson said. "For me, the most important thing will be the start of the game as was evident from our last two games.
"Performance-wise, there wasn't much difference in the games but the difference was in the first five to 10 minutes.
"It is important that we start the game against Hearts in the right fashion and be as positive as we possibly can.
"It will be a difficult game, but every game is - for all that these are the games you want to be involved in.
"We have worked really hard to get to this point of the tournament and we look forward to the challenge that we will face tomorrow night against Hearts."
St Mirren were well beaten in their previous semi-final appearance, against Rangers in the Scottish Cup last season.
MacPherson said: "This is a completely different game - different opposition and different venue.
"The result is the be-all and end-all of any semi-final and hopefully we get a positive one for St Mirren Football Club."
MacPherson's only signing on transfer deadline day saw Celtic's Graham Carey arrive on loan until the end of the season.
"Graham can play anywhere on the left side," the Buddies boss said.
"He has been in the Celtic system for four or five years now and had a loan spell at Bohemians last season where he did extremely well.
"We'll get him into the squad and into the way of working with things.
MacPherson added on his club's official website: "We would have liked another player in but we have competition for most areas of the park."
Source: Team Talk