September call-ups can be such a mixed bag. Occasionally, an organization's next great player does make their debut in the fall after knocking on the door all summer. More often, that player has broken down the door by July and become a vital part of the everyday lineup by the season's final month.
Cedric Mullins is not the type of transcendent player that a franchise will build around for a decade, but he seems to have a pretty good chance at being a solid regular in the big leagues for a long time. For the 2018 Orioles, in all likelihood, that is going to be the most exciting promotion of this season.