The 28-year-old was 4-0 with 11 saves and a 2.65 ERA in 46 games for Triple-A Lehigh Valley. While he will join the D-backs this weekend, the move was more designed to give the team some left-handed options next season.
"It's an area where we don't have depth," D-backs GM Kevin Towers said.
The D-backs currently have three lefties in the bullpen -- Joe Paterson, Alberto Castillo and Zach Duke -- but after trading Jordan Norberto to the A's in July, they don't have many lefties at the top end of the farm system.
Zagurski is out of Minor League options, so if he does not make the team next spring, he will have to be put on waivers before he can be sent down to the Minors.
Towers said the team was impressed by Zagurski's velocity, which can touch 94 mph, and the fact that he strikes out a lot of hitters. With Lehigh Valley, he fanned 63 in 54 1/3 innings. He also pitched 3 1/3 innings for the Phillies this year.
On the downside, command is sometimes an issue for Zagurski, who walked 27 for Lehigh Valley.
To make room for Zagurski on the 40-man roster, the D-backs designated right-hander Esmerling Vasquez for assignment.
"I didn't see him being on our club next year," Towers said of Vasquez, who spent most of this season with Triple-A Reno.