Clubs decided on an April 15 date to decide on plans for a merged 12-12-18 league structure during lengthy talks at Hampden on Monday.
The SPL needs an 11-1 majority to approve the plans, which would still need to be accepted by the Scottish Football League, but MacGregor has reservations over selling the new system to fans.
MacGregor's biggest concern is that fans might not buy season tickets when they do not know who their team will be playing when the top two divisions split into three groups of eight after 22 matches.
MacGregor said: "There is a lot of debate to go on yet and we will see how that goes.
"My concerns are for the fans. About 80 to 85 per cent of what is there I really believe in; one league body, redistribution (of money), more competitiveness, but I do feel for the fans who are asked to buy a 22-match season-ticket.
"That it is short-changing them and I believe that long-term, if that resolution is not sorted, it could be the end of people buying season tickets.
"The fan is the most important person, not the clubs, not anything else and I do believe that considering what the fans are getting is really important.
"But it is good, healthy debate. I must stress that and everyone in that room is very positive to get a resolution, so it goes ahead quite possibly."
SPL chief executive Neil Doncaster declined interview requests after the meeting.