From what I've read Kupp's setback was not expected and is an outlier in the typical recovery from the initial hamstring injury he suffered.
Because of the unusual nature of Kupp's injury and subsequent recovery, he - and possibly the Rams - thought it made sense to seek out some expert advise (and possible treatment) for his situation.
Personally I don't take this as a knock on the Rams' medical staff or on the treatment he received for the initial injury. It is quite likely that the Rams' medical staff followed the conventional protocol for a hamstring injury but Kupp has something going on that requires involvement from an expert. And I would not expect the Rams to have the leading expert in every aspect of medical science on their staff. My thinking is as great as the Rams' medical staff might be there is probably someone out there who qualifies as 'the top doctor' in treating hamstring injuries. So why not seek out that person?
AlbaNY_Ram