Matsuzaka underwent Tommy John surgery in May of last year and has been on rehab assignment in the minors. He's been reasonably effective in 37.0 innings across three different levels. In his last near-full season in the majors, 2010, Matsuzaka posted a 4.69 ERA in 153.2 innings. However, in the past he's proved that he can be both effective (2007) and very good (2008) on an extended basis.
Command has always been Dice-K's shortcoming, at least throughout his stateside career, and command is typically the last thing to return to a pitcher coming back from TJ. So there could be some fits and starts for a while.
In the nearer term, the Nats may be a somewhat accommodating match-up for Matsuzaka. Washington has been fairly ineffective against right-handed pitching this season, and they also swing and miss at a lot of pitches outside the strike zone. That might benefit Dice-K, who tends to live on the margins.
In any case, it appears that a rotation replacement for Daniel Bard has been found. For now, anyway.
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