Here is the announcement by WST:
Ronnie O’Sullivan has pulled out of next week’s BetVictor European Masters for medical reasons.
The World Champion was due to face Sean O’Sullivan in the opening round in Germany on the evening of Tuesday August 16th, but instead he has been replaced in the draw by Luke Simmonds.
It’s disappointing but it can’t be helped… I just hope that, whatever the issue is, it isn’t too bad.
Ronnie O'Sullivan 
No one can replace him. It’s a huge miss. All my prayers for him and for his prompt recovery! There are few gentlemen players like him.
Well said.
I also wish Ronnie a speedy recovery
He is such a wonderful player and a top guy
Hope Ronnie is back to playing again soon!! Hope whatever is wrong that it’s better soon
Does he have a problem with his right arm? Looked that way at the Championship League
I did have doubts he would go, even with the heldover match. When’s the last time he’s played in a ranking event in continental Europe?
2017 I think. That season he played in Germany and in Romania. But that’s not what happened here. The issue is real.
Is it something to worry about considering the tournament is in 10 days, so it’s not falling ill suddenly just before the competition.
Indeed Csilla.
I wonder if this is related to the arm problems he seems to have been experiencing lately…?