The key to Mendy's amazing crosses are the fact they're timed perfectly and hit at a lot of pace, especially the ones along the ground.
So it's not surprising that it'll take a few weeks/months to re-calibrate. You can see him getting the pace wrong a bit, but I will say he's put in some incredible aerial crosses that if our strikers weren't midgets would have been guaranteed goals.
Look at when KDB came abck last season. His passes weren't quite connecting, and he was out for a significantly shorter period of time overall. It really does take a bit of time to get your feet to do what your head is telling them to do.