My take is that he wasn't injured on the del Potro match. He was, as Darth noted way back, paying the price of the lack of practice. He was also obviously not confident enough, neither in his body, neither in his shots. Lack of confidence, in tennis, specially at this level, is a killler.
So, on one hand, there is a narrative behind his USO loss, but on the other... I don't think the word "injury" should be used on that particular match. He lost to a good player who was the better man that day, simple as that.
What will be always hard -- probably impossible -- for Federer fans to swallow is the abysmal difference in level when compared to just about any other tournament in 2017. If you rank his hard court tournaments from best to worst, USO and Montreal are neck and neck fighting to be the worst (and to me USO is the worst), while all the others are miles ahead. It is hard not to look for an explanation for such a huge discrepancy.