Here is an analysis from nfl.com:
Chang is a lean, but athletic passer with a very quick release. He is at his best in the underneath passing game, but needs to do a better job of setting his feet. He operates mostly out of the shotgun and has inconsistent mechanics and rarely employs the same throwing motion. Chang has the ability to recognize coverages, but is a marginal progression reader who carries the ball too low when on the move and this effects his accuracy. When he sets his feet in the pocket, he throws a tight spiral with good velocity. But when he tries to throw on the rollout, his balls sail or skip to the receiver.
He has the speed and balance to avoid the pass rush and buy time and will step up in the pocket, but will get a little antsy and not let the routes finish before putting the ball away and running with it. He has adequate arm strength to throw deep, but routinely makes his receivers adjust downfield. Chang plays with poise, but will flush too early and spends too much time locking on to his primary target rather than scan the field. He has a quick flick to release the ball, but his delivery and touch are erratic. Too often, Chang will force the ball into traffic, as he rarely seems to check down. While his release is quick, he just seems to lack solid throwing techniques (will release from the hips too much).
Because he is under constant pressure, he does not spend much time reading the defenses and can be fooled by disguised coverages. When this happens, he tries to rely on his foot speed and will either roll out and throw the ball up for grabs or leave the pocket too early and try to run with the ball. Timmy has good timed speed and shows athleticism running with the ball. He does a good job of securing the ball before running from pressure, but shows marginal accuracy throwing on the move.
Maturity issues arose in the past and he has been benched several times for ineffective play. He is a decent worker, but lacks the leadership skills and seems to shy away from the spotlight. He does not have great command in the huddle, and while he can throw the ball deep, lacks accuracy and touch. He will elude pressure with his lateral slide, but does not square his shoulders consistently to throw on the run. His frame might not be strong enough to withstand punishment at the NFL level, but he has little room to add more bulk.
Operating out of the shotgun, it is difficult to see if he has the foot quickness to drive away from center to get to his set point. In limited chances to see him line up under center, he looks like he drifts in the pocket. He might be better suited playing north of the border in Canada or in the Arena League. Chang could stick as a backup in the NFL, but he would have to operate in a West Coast offense, so not to expose his accuracy problems with the deep ball.
