While the reports are still unconfirmed, according to an article in Sport LivePR from November 18, Rosario had informed his winter league team that he tested positive in a recent MLB drug test. According to Frankie Thon, the general manager of the team Rosario plays for, Rosario reportedly took some pills during his arm rehabilitation that were on the banned list. Thon also said that Rosario would be allowed to play in the offseason league due to a new agreement between the MLB and the winter leagues.
Here is the explanation from the article through Google translation:
Rosario also said that the pills he took were they same kind he had taken in 2011 after he was struck in the face with a line drive while in Class A Beloit.
The Twins outfield/second base prospect, who was recently named as Baseball Prospectus’ seventh rated prospect in the system, his coming off a solid year in which he hit .302/.350/.460 with 10 home runs and 32 doubles split between New Britain and Fort Myers.
There is no official word on Rosario’s status from the Minnesota Twins.