He will definitely miss US Open.
I also believe that when he we fully recover he will come back much stronger. Djokovic said that he is suffering from his elbow for over an year - the exact time he started to lose more matches. He is still young and has at least 4-5 good years
Federer also suffered from injury and took some weeks rest and came back with full form and won the trophy. Djokovic also tremendous player yes he is still young, and he can have the capability to play at least 4 to 5 years. And have a chance to sit in top place in ATP ranking. He may miss US Open but no problem first he wants to recover from his injury.
2017 is a bad year for Novak Djokovic both physically and mentally. He will now be playing back in 2018 and I hope that in these months out there will be a good physical and mental form. Nadal and Federer have shown that is possible go back to playing the best tennis if in good shape.
Djokovic has great talent and is still young.
He can still win slam tournaments.