Duffy started at third base and went 0-for-2 with two walks in Wednesday's 5-1 win over Oakland.
The Rangers selected Duffy's contract from Triple-A Round Rock on Tuesday.
Duffy opted out of his minor-league contract with the Rangers on Sunday and is set to become a free agent, Jon Heyman of the New York Post reports.
Duffy signed a minor-league contract with the Rangers on Thursday and received an invitation to major-league spring training.
Duffy went 0-for-3 with a stolen base in Sunday's 4-3 loss to the Cubs.
Duffy went 1-for-3 with two RBI in Friday's 12-8 win over the Cardinals.
Duffy is starting at second base and batting leadoff Tuesday against the Tigers.
Duffy went 3-for-4 with a pair of doubles, a walk, one RBI and three runs scored in Saturday's 11-8 win over the Angels.
Duffy went 3-for-3 in Tuesday's 4-1 loss to the Blue Jays.
Duffy has been informed that he's made the Opening Day roster for the Royals, Anne Rogers of MLB.com reports.