Showalter said the team already knew about the bone spur, which Arrieta estimates he's had since at least since college, but the thought was that if doctors removed it now, it wouldn't disrupt the right-hander's offseason.
"There was a risk where if I don't have it done, going into [Spring Training] I'm fine, maybe May or June it starts to act up a little bit and I'd have to have something done," Arrieta said of the bone spur, which has never caused him any pain, just some soreness. "I just think it's the best route to where I don't miss any time."
Added Showalter: "[Arrieta] is in a real good spot, inning-wise. He's healthy. One of the first things he heard is guys taking about how good the ligament looks and everything."
The procedure could actually help Arrieta -- who went 6-6 with a 4.66 ERA in 18 starts this year for Baltimore -- and his performance in the long run. The 24-year-old said he was told he would have improved range of motion and could add a tick of velocity once he's fully recovered, which is expected to be around mid-October.
"In the big picture of things, it's not that disappointing," said Arrieta, who credits cleaning up his mechanics for his solid end-of-season numbers. "I would have liked to make that start [on Tuesday at Boston], but if it's going to be beneficial for my career and my future to shut it down and have this done now, then I'm all for it.
"It's not worth jeopardizing your future over one start."
Asked to reflect on his rookie season, Arrieta said he would call it a success, and Showalter -- who took the reins on Aug. 2 -- agreed.
"You can see why people are high on him," Showalter said of Arrieta, who tossed six scoreless innings in his final outing. "I can see why he's had challenges at times, too. ... He knows he's going to come in next year and he's got a chance to make our club."
Tuesday's start at Fenway Park will now go to either long man Rick VandenHurk or Wednesday's starter, Brad Bergesen. The O's are already working with a six-man rotation, so Bergesen wouldn't have to pitch on short rest and Showalter said much of the order of those two depends on if VandenHurk is used in Saturday's game against the Yankees or not.