Hellickson, 27, has been the subject of trade rumors all week after a report on Monday noted that the Rays were working on a deal to send him to a National League team. He joins a D-Backs' rotation that is sure to include Wade Miley, Josh Collmenter, Bronson Arroyo and Patrick Corbin next season, with Chase Anderson, Vidal Nuno and Andrew Chafin also in the mix for spots.
Hellickson has struggled over the last two seasons and was injured for most of 2014, starting only thirteen games and posting a 4.52 ERA. He posted two straight excellent seasons in 2011 and 2012 for Tampa, winning the AL Rookie of the Year award in 2011 after posting a 13-10 record and 2.95 ERA in 29 starts.
Williams, a second round pick in the 2013 draft, split the season between Rookie-level Missoula and Single-A South Bend, hitting .351/.403/.467 with four home runs and 46 RBI in 74 games. The 19-year old was ranked by MLB.com as the fourteenth best prospect in Arizona's organization after the season, with his power potential being the biggest plus. He recently converted to the outfield after developing as a shortstop, but the expectation is that he will become an adequate corner outfielder at the major league level.
Velazquez, 20, was the Diamondbacks' seventh round pick in the 2012 draft and hit .290/.367/.428 with nine home runs and 56 RBI for Single-A South Bend last season. MLB.com ranked him as the twelfth best prospect in Arizona's system after the season, citing an improved plate approach that caused him to reach base in 74 consecutive games last season.