The Red Sox announced Thursday they have activated right-hander Lucas Sims from the 15-day injured list.

To make room on the active roster, the club optioned left-hander Bailey Horn to Triple-A.

Originally acquired from the Cincinnati Reds, Sims was one of two relievers the Red Sox added at the trade deadline to bolster their faltering bullpen. The 30-year-old so far hasn’t made an impact, and prior to his injury he recorded an 8.10 ERA in 11 games with Boston, allowing nine earned runs in 10 innings before going on the injured list Aug. 27.

Sims has since missed three weeks but threw two scoreless rehab outings in Worcester. Luis Garcia, the other reliever added at the deadline, is also in Tampa Bay but hasn’t yet been activated.

Among those who have attempted to pick up the slack in Boston’s bullpen has been Horn, but the rookie left-hander has struggled throughout his tenure in the majors. So far Horn has posted a 6.50 ERA in 18 games with the Red Sox, allowing 13 earned runs on 10 walks and five home runs over his 18 innings of work.