He was signed by the Yankees as an amateur free agent in 2006, and by 2009, Baseball America had already recognized him as one of the 100 best prospects in the nation. His peak season came following the 2010 season, when he was ranked as the 3rd best prospect in baseball after hitting .289/.353/.517 in 504 plate appearances at the Yankees Triple-A affiliate.
Montero, along with Hector Noesi, was traded to the Mariners for Michael Pineda and Jose Campos (minors) in 2012. He hit an unimpressive .260/.298/.386 that season as the main designated hitter, his only with a full 500+ big league plate appearances.
Kendrys Morales came to Seattle to handle the DH in 2013 and Mike Zunino was their main catcher, leaving the then-23-year-old Montero in the minor leagues for most the season.
By this time, Montero had lost most of his average and power hitting abilities and had surgery on a medical meniscus in his knee on June 5. Plus, he was suspended for using PEDs. He had begun the fall down the roller coaster and the Mariners had lost their patience for him and his abilities.
Then, last Spring Training, Montero arrived 40 pounds overweight. That was considered the end of his "prospect" talent. Last season, he spent almost the entire year with Triple-A Tacoma, and didn't even register 500 plate appearances there. Montero was suspended again, this time due to an altercation with a club scout. Practically everyone had given up on him.
This offseason, Montero worked to get himself back into shape, coming back to Mariners camp looking almost like a different person. He came into camp at 230 pounds, a weight he hasn't been since 2011. It may or may not help his performance this season, but good for him to take the initiative and get himself back into shape. I'm definitely looking forward to seeing what he can do in Spring Training.
Here's the transformation of Montero from last Spring Training to this Spring Training.