Reigning motocross champion Ryan Villopoto will not defend his title this summer because he is having surgery on his left knee.
Motocross is off-road motorcycle racing; the 12-round Lucas Oil Pro Motocross Championship series opens May 24 at Glen Helen Raceway in San Bernardino.
The stadium version of motocross is supercross, whose season runs from January to early May, and Villopoto just won his fourth consecutive supercross championship.
Villopoto's knee was bothersome during the supercross season and the Seattle-area native opted to get the surgery "to fix a lingering injury," Villopoto's Irvine-based Monster Energy Kawasaki team said Monday.
"To not be able to race this summer [in motocross] is very disappointing," Villopoto, 25, said in a statement.
Reid Nordin, the team's senior manager of racing, said "we are listening to [Villopoto's] medical team to make sure he gets his knee fixed properly and he can compete for championships in 2015."
The team said rider Brett Metcalfe would replace Villopoto and join teammate Jake Weimer for the motocross season, which ends Aug. 23.