That's a fair point. But Ellsbury is 27 and in his peak years. Realizing extra power is not surprising here. I don't think he's going to be a consistent 25 HR hitter, but 15-18 seems reasonable. Reddick is 23 and has very limited AB in the MLB, especially against elite pitchers (i.e. Verlander, Haren, Weaver, Sabathia, Lee, Hamels, Halladay, Oswalt), all of which could potentially be seen in October.
I hope this is not construed as me not liking Reddick, or not rooting for him. I just think it's a fairly legitimate concern based on his small sample size in the MLB and his lack of experience against elite stuff at the MLB level.