Texas Rangers ace Yu Darvish suffered from a nerve problem in his lower back and buttocks area from the beginning of September through the end of the team's season, ESPN reported Wednesday.
The issues caused numbness in Darvish's right leg, ESPN quoted sources familiar with the matter as saying.
Darvish failed to finish the sixth inning in each of his last three starts, the first time that has happened in two seasons with the Rangers, and his fastball velocity was down.
Darvish, who finished his second season in the majors with a 13-9 mark, had 277 strikeouts to lead all major league pitchers.
Texas manager Ron Washington acknowledged Tuesday that right-hander Darvish had physical issues late in the season, but would not reveal what they were.
He also praised the former Hokkaido Nippon Ham ace's contribution to the team this season.
"It was an amazing statistical year. Darvish is a tremendous pitcher and this was a learning experience for him again.
"I certainly expect for him going into next year to take it to another level as far as his development and progress goes here in the United States."