Lorenzo was neck and neck with Ducati's Andrea Dovizioso for 15 of the 22 laps before he developed a problem with his helmet in the closing stages of the race.
Despite struggling to see, Lorenzo finished in fourth place, while his team-mate Valentino Rossi ousted Dovizioso in the final stretch to win the race.
"I was feeling better and better, but suddenly there was something wrong when in the top part of the helmet the foam came loose and came down so I lost half of my vision," Lorenzo told reporters. "I couldn't see the corners very well and couldn't concentrate or ride like before and I was losing half a second or more per lap.
"In a race where, at minimum, I could fight for the win, I finished fourth."
Prior to his helmet problems, Lorenzo admitted that his bike had a hard time keeping up with the new and improved Ducati that Dovizioso was riding. However, despite this, Lorenzo was confident about his chances of beating Dovizioso.
"I was faster and I passed him where I could in the fast corners," he said. "On the [top] speed and in braking the Ducati was much stronger than our bike, but we could recover on corner speed while the acceleration was similar.
"To be honest, I thought that I could win the race, though it surely wasn't going to be easy, because Valentino had a very strong pace at the end of the race."