Number of minutes played has nothing to do with stamina. It used to, but that was changed at the start of last season.
Stamina drops for all players, but it drops faster in players with higher stamina. A drop from Average to Mediocre probably means that it was a low Average before, and is a high Mediocre now.
It appears that you will be able to maintain stamina levels with one week of stamina training per season. You might want to consider doing some stamina training the next few weeks since as an 8th place team you will only have 2 games/week for the next 3 weeks and stamina training does not require minutes played.