Magnus say's that Robin has experienced slight problems with his knee on and off for a while now, and that it got worse during IW and Miami (which isn't too surprising, as the surface is tough on the body, and he played many matches). He's swollen in the knee joint due to excessive strain. They have consulted doctors which says there should be no risk of aggravating it if he rests and allows it to heal.
It shouldn't be too worrying. Apparently Robin started feeling better just a couple of days after he got back to Europe.
Magnus also says that a key goal this season is to arrive well prepared for every tournament, so they might have thought about skipping MC anyway as it's a new surface. They will travel down to Barcelona a week in advance to train.
Oh, yeah, they are very pleased with how well Robin has done so far this season. He's feeling good about himself and his results, but is aware that there are many now who really wants to beat him. They stress on not underestimating any opponent.