The world number one, who was knocked out in the quarter-finals of the Monte Carlo Masters on Friday by fellow Spaniard David Ferrer, sustained a back injury at the Australian Open in January.
"I don't have to lie to anybody. After what happened in Australia it was little bit harder for me to find again the intensity, the confidence, the inside power that I always have," said Nadal after a 7-6(1) 6-4 defeat by Ferrer ended his 30-match winning run on clay.
Nadal made an uncharacteristic 44 unforced errors against Ferrer on centre court, failing to find the right length as Ferrer never released his grip.
Asked if a back problem was responsible for his under-par performance, he replied: "No, no, the back is not an excuse.
"No, the back is in good shape. The physical performance is fine. No problem about that."
After losing the Australian Open final to Swiss Stanislas Wawrinka, Nadal won the Rio de Janeiro tournament on clay before being eliminated in the third round of the Indian Wells Masters and losing in the final of the Miami Masters to Novak Djokovic.
"Even if I won Rio, I played the final in Miami, it remains something in my mind and in my game," said Nadal.
"I'm going to fight to try to find that solution soon."
Nadal plays in the Barcelona Open next week, a tournament he has won every year since 2005, apart from 2010 when he did not take part.
The French Open, which Nadal has won a record eight times, starts on May 25.
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