Broxton, 29, had been out for a little more than three weeks with a strain in the flexor mass of his right elbow. He's battled numerous elbow problems in recent years, ranging from general soreness to surgery to remove a bone spur and loose bodies.
In 29 appearances before the injury, Broxton had a 4.33 ERA with a career-low 6.7 K/9 in 27 innings. His trademark fastball velocity has very gradually been trending downward according to the PitchFX data at FanGraphs. He is averaging 93.9 mph with the fastball this year after sitting 97.5 mph back in 2009.
Cincinnati signed Broxton to a three-year, $21 million contract this past offseason with the intention of making him their closer. Aroldis Chapman was expected to move into the rotation, but the team instead stuck him back in the closer's role during spring training, making Broxton a well-paid setup man.
Villarreal, 25, allowed eight runs in 5 2/3 innings for the Reds. He made one start and one relief appearance.