They can, although it's not very common that it happens. They can also lose multiple levels of gameshape in a week due to very few or way too many minutes. The random factor always applies.
I just checked my data for the last couple of weeks. Since the start of this season my players had a total of 82 game shape changes and 3 times did it improve by two levels (59, 70 and 74 minutes) and once (98 minutes) it fell by 2 levels.
All these 4 changes were from 7 to 9 respectively 9 to 7.
We have both said a lot of things that you are going to regret.