The Arizona Fall League is a fall league that teams use to send players who they feel might need a little extra work after the season is over and this year the Yankees are sending four players – David Phelps, Corban Joseph, Ronnier Mustelier, and Rob Segedin (source).
Phelps, 24, is the biggest name on this list. He is practically major league ready, but he needs to add innings after missing time this season with a shoulder injury. He has a 3.16 ERA, 7.3 K/9, and a 2.2 BB/9 over 108.1 innings this season. His career high innings was 158.2 from last season and the Yankees are trying to get him back to that point.
Joseph, 22, is a good hitting second baseman for the Yankees. He is nearly major league ready and they are probably just sending him to get him even closer and to showcase him a bit. As a second baseman he is stuck behind Robinson Cano making it likely he is traded in the future. He had a .347 OBP and a .750 OPS in 124 games in Double-A this year.
Mustelier, 27, is a Cuban born athelte who was just signed this year. He has torn the cover off the ball in Single-A, putting up a .421 OBP and a .974 OPS. He also has played five positions so the Yankees probably want to get a better idea of what they have in him.
Finally there is Segedin, 22, last year’s 3rd round draft pick. He did well across Single-A this year putting up a .360 OBP and a .771 OPS. He is an infielder listed as an outfielder on the AFL roster which probably means that the Yankees are sending him there to transition into a full-time outfielder.
There is a possibility that the Yankees could send one or two more players, but at this point it seems unlikely.