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The bad news with Darvish is that his mechanics are generally excellent either than the timing flaw. His delivery relies on a lot of athletic ability so I wouldn't expect him to last into his 30s, but its possible he could last 4-6 strong years.
That said, Darvish will likely eventually stumble into elbow issues. Its a matter of when.
CJ Wilson is a much bigger injury risk. I like that swap for Texas.