I can certainly remember cases where players were initially ruled "questionable to return" and ended up on IR 2 days later.Michel was initially announced as "Questionable" to return, right? My understanding is that they know ACL or Achilles tears right away, so that would suggest it wasn't one of those.
While I believe that most ACL and Achilles injuries can be diagnosed on the sidelines, I don't think that applies to all such injuries. And the medical staff will want to see some sort of confirmation through imaging and other tests before making any definitive diagnosis regardless.
Sorry for the pessimistic tone.