Assistant changing goalkeepers in Instant Result

[Greyfriars Bobby]

First of all, I apologize in advance if someone's asked about this already.

I'm using a match plan that allows substitutions after the 70th minute. I've used Instant Result for several matches, and almost every time, my assistant has brought on a substitute goalkeeper. It doesn't seem to relate to the 'keeper's performance; once, he was taken out when he had a clean sheet, and once he'd allowed one goal. The 'keepers haven't been tired, either, and it didn't appear that they were sick.

Is there a way to make sure the assistant will still sub for tired or ineffective players, without choosing to remove a goalkeeper who's doing fine?

Thanks!