KANSAS CITY -- The Royals are shutting down Brady Singer for the remainder of the year after the right-hander experienced back tightness following Monday¡¯s start against the Guardians.

Singer, 27, has had an up-and-down year and struggles in September. He skipped a start at the beginning of the month because of arm fatigue and has seen a velocity decrease in his last several starts. On Monday, when he gave up four runs in six innings to the Guardians, Singer¡¯s sinker averaged 90.3 mph, 1.8 mph lower than his average velocity this season.

The move comes among a flurry of them on Tuesday with Singer and reliever Brad Keller (symptoms associated with thoracic outlet syndrome) being placed on the 15-day injured list.

Pitchers Jonathan Bowlan, the Royals¡¯ second-round pick in 2018, and Anthony Veneziano, the club¡¯s No. 16 prospect, were promoted to join the pitching staff. Manager Matt Quatraro said the two will pitch out of the bullpen to begin, and the Royals will figure out who will start in place of Singer the rest of the year based on bullpen usage.

To make room for Veneziano on the 40-man roster, the Royals designated infielder Matt Beaty for assignment.

Singer¡¯s back pain, which is officially diagnosed as a left lower lumbar strain, has been an on-and-off issue. But it flared up more than usual after Monday, which he reported to the team.

¡°Kind of a whole collective agreement of, ¡®Why keep pushing it,¡¯ if it¡¯s going to get worse,¡± Singer said. ¡°We¡¯ve done all the testing on arm, back, everything, and it¡¯s all healthy. That¡¯s the positive that everything is fine.¡±

The Royals have maintained that nothing is wrong with Singer¡¯s arm after being asked about the velocity decrease several times. When he was shut down for 10 days following his start in Seattle on Aug. 25, he went through a battery of exams, including an MRI, that showed nothing out of the ordinary with his shoulder or elbow, team officials said.

¡°We wouldn¡¯t put him on the field if we didn¡¯t feel like he could compete,¡± general manager J.J. Picollo said. ¡°We¡¯ve seen the drop in velocity, his performance hasn¡¯t been as good, and you get to a point and say, ¡®Do we want to keep pushing through the soreness he¡¯s had?¡¯ The thing we are confident about is that there¡¯s nothing wrong with his arm at all.?

¡°And if the circumstances were different, he would still be pitching. He did not want to stop pitching. ¡­ But knowing where we are, let¡¯s do the thing that¡¯s right for your career and for us.¡¯¡±

Singer ends his fourth year in the Majors with a career-worst 5.52 ERA across a career-high 29 starts and career-high 159 2/3 innings. He had stretches of starts where he looked like the pitcher he was last year, when he posted a 3.23 ERA, but he could never find consistency and fought mechanical struggles all year.

¡°Obviously, my season stats aren¡¯t good,¡± Singer said. ¡°Not happy with those. But you sit here and learn. You learn stuff in the big leagues each year. Something I can take into next year is just figuring out ways to get better throughout the offseason.¡±

Singer would have been the Royals¡¯ Opening Day starter if not for his participation in the World Baseball Classic for Team USA, an experience he¡¯ll never forget -- but one that disrupted his normal Spring Training. He ramped up earlier in the winter to be ready but then only logged two innings in the tournament.

When he returned to Royals camp in late March, he had to ramp up quickly to be ready for the season.

¡°I¡¯m not going to make excuses because I would do it again,¡± Singer said. ¡°Playing for that team was unbelievable, but it was an earlier buildup and then not a normal Spring Training, which could have led to fatigue.¡±

While it will never be known whether there¡¯s a true link between the WBC disruption and how Singer is feeling now, Picollo said it¡¯s something the Royals will learn from.

¡°Reflecting on it, it makes sense why we¡¯re seeing some of the things we¡¯re seeing,¡± Picollo said. ¡°We¡¯ll never stop a player from wanting to go participate, but I think there needs to be deeper discussions about their role on this team and what the ramifications could be by disrupting the spring. But it was a full year.

"In some ways, he may have had to work harder this year than any other year he¡¯s pitched because of the abnormalities he¡¯s gone through.¡±