It make no difference how fast or slow Novak's serve is normally.
If his shoulder were significantly injured, his normal serve speed would be significantly decreased.
It has not. His top speed was 124 in the first set and 121, in the 4th set of the last match.
With a shoulder injury, I would expect his top speed to go down to at least 110, lower if the injury is more severe..
His avg 1st serve would go from 114 to at most 105,
One just can't keep serving normal speed with a shoulder injury.
He got the MTO after being broken, and then won every game from that point.
It was a momentum breaker. After the match he said he was fine.
Aces are a result of serve speed (influenced by conditions), placement, and the returner's ability to return.
Novak has never been known as an ace machine, though he has improved his serve some in the last couple of years.
More double faults can come from nerves, pressure, exhaustion, or simply when a player hasn't played a lot of matches and is inconsistent.
I think with Nole, recipient of a walkover and 2 retirements, and not a lot of play since Roland Garros, it is mostly the latter.
Respectfully,
masterclass