Wishere was barely 18, so Raheem has still one year to wait. IMO, he isn't physically ready and lacks both the technique and the maturity to cope with first team football. Wishere were playing in a very different position, and already had the vision a central midfielder needs.
And I know, "Sterling" and "lacking the technique" are quite strange when associated together. His game is based on speed and technique, and the very first won't be an asset anymore in the PL, while the second won't be enough. One year may give him some time to improve his physical ability, and become a reliable first team footballer. Now is too soon.