According to Ken Rosenthal and Matt Gelb of The Athletic, "Arrieta has a bone spur in his right elbow, according to a source, and it likely will require surgery to remove it—surgery that those involved had hoped could wait until the end of the season. That delay is no longer certain."
According to that report, the Phillies "were aware of the bone spur in Arrieta's right elbow" and have "monitored him while Arrieta has managed his work, falling back on the lessons he learned before pitching through a similar ailment."
Arrieta's numbers started slipping little by little following his Cy Young Award-winning season in 2015, which led to some questions about what kind of pitcher he would be for the Phillies in the first year of his three-year, $75 million deal.
That general trend continued for the veteran right-hander in 2018. He finished 10-11 with a 3.96 ERA and 4.26 FIP in 31 starts, per FanGraphs. His 1.9 WAR were his fewest since 2013, the year before his breakthrough with the Chicago Cubs.
Arrieta's production is still going in the wrong direction this year. Through 17 starts, he's 8-7 with a 4.43 ERA and a 5.14 FIP.
The Phillies sent a big message to the rest of the league when they signed Bryce Harper to a 13-year, $330 million deal in the offseason. Although a World Series title isn't the minimum bar for success, Philadelphia clearly has championship aspirations.
Losing Arrieta for an extended stretch would be a tough blow since the Atlanta Braves have the pieces to once again claim a National League East title.