The trade, announced Friday night, is the first major move by new Diamondbacks General Manager Dave Stewart. The Diamondbacks sent Tampa Bay two minor league prospects, infielder Andrew Velazquez and outfielder Justin Williams.
"Kind of shocked a little bit," Hellickson said in a phone interview with the Tampa Bay Times. "Mixed emotions."
Hellickson, the American League Rookie of the Year in 2011, has a 40-36 career record and a 3.78 ERA in 115 appearances, 108 starts, in five seasons with the Rays.
He began the 2014 season on the disabled list and went 1-5 with a 4.52 ERA. Hellickson won a Gold Glove Award in 2012.
Stewart said at this week's general manager meetings that acquiring a starting pitcher was a top priority. Hellickson had been mentioned prominently in trade rumors at the meetings, with Milwaukee considered a possible destination.
"I'm definitely excited to get over there to Arizona and get back to the pitcher I was," Hellickson told the Times. "I'd heard enough rumors so you kind of expect something, but I'm definitely kind of shocked. I'm definitely going to miss a lot of people over there."
The Diamondbacks are coming off an awful season, finishing with the worst record in the majors at 64-98. Manager Kirk Gibson was fired and general manager Kevin Towers reassigned. Towers subsequently quit and Tony La Russa, who took over during the season as head of baseball operations, hired Stewart.
The 27-year-old Hellickson, 6-foot-1 and 185 pounds, started out well after coming off the DL last season, going 1-1 in five starts with a 2.03 ERA, but struggled the rest of the way.
He won the rookie of the year award after going 13-10 with a 2.95 ERA in 2011. He is arbitration eligible this year and would become a free agent in 2017. He earned $3.625 million on a one-year contract last season.
Hellickson had a career-high 135 strikeouts in 2013, going 12-10 but his ERA was a career-worst 5.17.
With three young shortstops at the major league level -- Chris Owings, Didi Gregorius and Nick Ahmed -- the Diamondbacks decided to part with the 5-foot-8 Velazquez, an intriguing prospect who reached base in a minor league record 74 consecutive games last season for the low Class-A South Bend Silver Hawks. The 20-year-old, who also plays second base, hit .290 with 18 doubles, 15 triples, nine home runs, 56 RBIs and 50 stolen bases in 134 games.
Velazquez was named the Midwestern League prospect of the year.
The 19-year-old Williams hit .351 with 12 doubles, five triples, four home runs and 46 RBIs in 74 games with rookie league Missoula and South Bend.
The Associated Press contributed to this report.