Signed as an amateur free agent out of the Dominican Republic, Montero’s professional career launched in 2011 with brief progressive appearances in the Dominican Summer League, with Kingsport of the Appalachian League, with the Gulf Coast League Mets, then finally with Brooklyn of the (A-) New York-Penn League. Rafael made 17 combined appearances, posting a 2.15 ERA and 0.958 WHIP, with an 8.4 K/9 and 1.6 W/9 average over 71 innings pitched.
In 2012, he continued skyrocketing through the system. Montero made 20 combined starts split between (A) Savannah and (A+) Port St. Lucie. He posted an overall 11-5 record with a 2.