He's only 23 and has his half-decade of peak productivity ahead of him.
Although I wouldn't shed too many tears if he left for a decent fee, I'd say if we think he's average now based on performances since he was 18, we should definitely re-sign him because his value is going to rise.
I believe he will be injured less and settle more profoundly into his craft over the next five years.
For comparison Bergy, Ljungberg and Henry moved here at around 26, 21 and 22 years of age respectively. Those guys were in or entering their peak years when they arrived here.