He was finally noticed by professional scouts when he was chosen as a member of the 2000 Sydney Olympics Japanese national team, where he marked a win in a game against Italy.

He pitched in the Japanese national amateur baseball tournament in late 2000 and was drafted in the 4th round by the Chiba Lotte Marines that year.

He was chosen as a member of the World Baseball Classic team in 2006, but pitched poorly during the season, ending up with a 5–11 record, and a 4.35 ERA.

On November 6, 2004 David Ortiz of the Boston Red Sox hit a 525-foot home run off Watanabe when the United States Major League Baseball team faced the Nippon Professional Baseball team in the second game of the traditional Japan All-Star Series.

He was released by the Red Sox on March 30 and on April 14 signed with the Lancaster Barnstormers of the Atlantic League of Professional Baseball.

Batters have trouble timing their swings against Watanabe, because his pitches seemed to come in at completely different speeds compared to those of conventional pitchers.

Watanabe's gyroball was very slow, clocking around 60 mph, somewhat similar to the curveball of Washington Nationals pitcher Liván Hernández (which actually was more like an eephus pitch).