According to this article on tennistalk.com (by Johan Lindahl), Rafa’s knees will be fine in a few weeks. No mention of the source for the quotes from Dr. Cotorro, though.
“Rafa had tendinitis in knees for some time, the situation is a bit better now, but that was not enough for Wimbledon”, said Dr. Ángel Ruiz Cotorro. “With rest and recovery, he will be ready in three to four weeks for tournaments”.
“A fortnight of big efforts would be too much for him”, said Ruiz Cotorro. “Grass is very demanding surface and he is aware that this was the best for him”.
“Big clay court tournaments are crammed into the spring, and Rafa had no time to rest. But when he steps on the court next time, he will be like new”.
Thanks to patzin for making sure I saw this.