Nadal had not trained in two weeks until four days ago because of the issue and the Spaniard was forced to withdraw from last month's Miami Masters semi-final against Andy Murray.
"I couldn't move my knee. I have to try to be patient," said Nadal.
When asked if he found it easy to adapt to playing on clay again, he said: "I don't feel comfortable even today. I need time. I need to spend hours on court, play matches and I need practice.
"I had to stop after Miami for a long time. I started to practise four days ago, so it's not enough."
Nadal hasn't won a tournament for 10 months but has been victorious in every Monte Carlo Masters since 2005, claiming seven consecutive titles.