So after almost a whole season is in the books, it's hard to believe a 22 year old who has had a limited amount of innings until now may not have some down velocity simply due to some fatigue?? Here was a scouting report with quotes from Sherman when the original trade was made. Whalen was considered the more heralded of him and Gant at the time. Sherman seemed to think he hit 92-93 in the minors and he would have known.
http://m.bleacherreport.com/articles/2530864-rob-whalen-john-gant-to-braves-trade-details-scouting-report-reaction
Ok, then let's assume he has lost a couple ticks due to fatique. That gets him to an average sinker when he is well rested. I still don't see a "very good sinker" in there.