Walker replaces pitcher Kevin Gausman, who was optioned to Norfolk following last night's 8-3 loss to the Twins.
Walker, 24, batted .266/.321/.380 with 20 doubles, five home runs and 28 RBIs in 78 games. He's hit safely in 12 of his last 14 games, batting .328 (20-for-61) with three home runs, seven RBIs and five multi-hit games.
In 85 at-bats this season against left-handed pitching, Walker had a .306/.348/.482 slash line with 26 hits, three home runs and eight RBIs. He could be in today's lineup against Twins left-hander Tommy Milone.
Walker, a fourth-round pick in the 2012 First-Year Player Draft out of the University of South Carolina, was named the Orioles' minor league Player of the Year last summer after leading the farm system in home runs (26) and RBIs (96). His major league debut came on Sept. 17 against the Blue Jays, and he homered three days later against the Red Sox.
Walker went 3-for-18 with a double, home run, walk and nine strikeouts in six games with the Orioles. He's wearing No. 34 today.