Games shape is supposed to drop beyond about 90 and below 40 or so, but not too much outside those limits. You'll be OK as long as it doesn't happen on a regular basis week after week.
Where you will see the biggest drop in game shape is if a player is injured for a few weeks and doesn't get any PT. Though it might not drop much as 36 minutes, it won't recover very quickly either.
About of time played has no effect on stamina, at least on a weekly basis. If a player has low stamina the more minutes he plays in a game, the worse he will perform as he goes on.