Blum only had 11 at-bats to start the season before going on the DL on April 18 with a strained left oblique. In 12 rehab games for Triple-A Reno, the 39-year-old hit .237 with three doubles, a homer and eight RBIs. In eight games with the D-backs this season, Blum was 1-for-11 with an RBI.
Blum joined the team in Phoenix on Monday and despite not being in the starting lineup, was available against the Padres.
"If the game calls for it, he'll pinch-hit for sure," D-backs manager Kirk Gibson said. "We've been short with McDonald out, that puts a strain on what you can do to create a matchup. With Blum and a full-roster, we can plan ahead."
McDonald was originally in the D-backs' lineup on Saturday in Milwaukee, but was scratched beforehand. Because he hasn't played in more than a week, the 37-year-old could only miss another week's worth of games if all goes well in his rehab.
"He came to me on Saturday and said he couldn't play," Gibson said. "It had been bothering him."
McDonald was batting .267 with seven doubles, four homers and 12 RBI in 33 games this season.