Well, I haven't checked your work, but it's certainly possible that you can't narrow it down any further with the ratings. This happens occasionally, most often with relief pitchers.
One thing you can do is actually pitch the guy a few times. Once he's fatigued, if you hold your mouse over his fatigue number in the manager's page, you should see a pop-up that says something like "on pace for 12% more pitches than actual." From that you should be able to work out the number of innings your guy actually threw in real life (assume he threw 15 pitches per real-life IP). Which should allow you to ID the exact player.