Tommy John Surgery is an entire reconstruction of the UCL and requires most a year to recover from. Some have come back quicker, and keep in mind that most are pitchers. So Bryan may be able to come back sooner than others.
The fact that we are going into September means, however, that even on a fast healing body the UCL would not heal in enough time for him to be back in January. 3 months is too soon to recover from such a surgery. If not fully recovered, he could not get cleared to wrestle as he would risk needing yet another surgery. Keep in mind, wrestling is about NOT intentionally hurting someone in the ring. The person in the ring is your partner, not your enemy. Due to that, he risks not only doing damage to himself but to someone else in the ring and that is a big no-no.
Clearly this is a blow for the WWE, as having Bryan out even longer would not help them. They know he is popular, so it is very doubtful the WWE would release him for being injury prone as his is the first time he has been drastically hurt. The main issue the WWE knew going in was that Bryan was nursing random injuries coming off of 10 years of wrestling from the Independent scene. WWE feared correctly as one surgery led to another, but the cool part is that it seems that the Tommy John Surgery may be the end of it.