Last season, Crichton, 26, allowed 11 earned runs in 12 1/3 innings for the Orioles and went 7-2 with a 3.02 ERA in 47 2/3 frames for Triple-A Norfolk. He has a career 3.46 ERA in 141 career Minor League outings.
"He's got a little power in his fastball -- mid 90s, got a little bit of a different look, a little lower [arm] slot," D-backs GM Mike Hazen said of Crichton. "He's got a good breaking ball. We've evaluated him for a while, and we like the contrast in styles and think he fits in depth-wise with what we're trying to do."
Crichton has Minor League options left this year and next, which is important because teams typically need to dip into their farm systems for bullpen help during the season, either due to injuries or workload.
"As we need relievers through the course of the season, I'm hopeful he'll be a dependable guy that we can call upon to go with the group down there," Hazen said.
Tomas was signed to a six-year, $68.5 million contract in December 2014. The 27-year-old hit .272/.313/.508 with 31 home runs and 83 RBIs in 140 games for the D-backs in '16, but he played in just 47 contests in '17 due to injury.
The outright assignment does not really change Tomas' standing with the organization. He's still starting the season at Triple-A, but the D-backs will have to put him back on the 40-man roster if they decide to call him up.
Injured outfielder Steven Souza Jr. (strained right pectoral muscle) played catch from 90 feet on Monday, which is a big step forward for him because throwing was believed to be the thing that stressed his pec more than anything.