Chiba Lotte Marines Watanabe made his debut in April 2001, starting a game against the
Orix BlueWave. He won his first professional game with a complete-game shutout, and ended the season with 2 wins. In 2002, he pitched in 6 games and had a record of 0–3. In 2003, he gave up 8 runs in his first start, and became a part of the starting rotation at the middle of the year, going 9–4 with a 3.66 ERA. He won 12 games the following year, won 15 games in 2005 with a 2.17 ERA. The Marines won their first championship in 31 years in 2005, and Watanabe pitched in the second game of the Japanese championship series against the
Hanshin Tigers, giving up 4 hits in a shutout victory. 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. He also led the league in hit batsmen (14). He holds the Japanese record for skipping stones, recorded on a show on
Nippon Television.
Chiba Lotte Marines manager
Bobby Valentine made an appearance on the show as well. 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. This blast by "Big Papi" has been recorded as the longest home run ever hit at the
Tokyo Dome. Watanabe was released by Lotte on November 4, 2013.
Lancaster Barnstormers On December 13, 2013, Watanabe signed a minor league contract with
Major League Baseball's
Boston Red Sox. He was released by the Red Sox on March 30, 2014. On April 14, 2014, Watanabe signed with the
Lancaster Barnstormers of the
Atlantic League of Professional Baseball. He made 39 appearances (16 starts) for the Barnstormers, posting an 8-2 record and 3.37 ERA with 91 strikeouts and one save across innings pitched. Watanabe returned to Lancaster for the 2015 season, compiling a 7-5 record and 2.94 ERA with 75 strikeouts in innings pitched across 21 games (18 starts). ==Pitching style==