The team offered no further information on Tazawa's condition. He had not been seen much at camp since he gave up four runs in an inning against Tampa Bay on March 22.
Tazawa, who was signed as a free agent after pitching in Japan's amateur leagues, had allowed five home runs and eight runs overall in just seven innings this spring.
The 23-year-old was 9-5 with a 2.57 ERA in 18 starts at Double-A Portland in 2009. He appeared in two games with Triple-A Pawtucket and then six games and four starts for the big club.
Tazawa, a righty, was 2-3 with a 7.46 ERA for Boston.
At the time he was signed, Tazawa had pitched exclusively out of the stretch and was taught how to throw from the wind-up.
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