I've done some reaserch and am revising my expectations.
JJ Redick's 2nd season:
23 years old. 8mpg, 4.1ppg, .444/.395/.794
Aaron Afflalo's 2nd season:
23 years old. 16.7mpg, 4.9 ppg, .437/.402/.817
Danny Green's 2nd season:
23 years old. 11.5mpg, 5.1ppg, .486/.368
If he winds up having the career of any one of these guys, I think he'll have been a good pick by the Celtics. All of these guys were much older in their 2nd seasons and didn't have terribly impressive 2nd seasons and all of them were traded by their original teams before they fully developed.