The Cubs moved Arrieta closer to the third base side of the rubber and encouraged his preferred delivery, in which he steps far to his right before firing. His fastball seems to have more run from this angle, but his raw stuff was already so impressive that Cubs reliever Tommy Hunter, a former Baltimore teammate, called Arrieta the most talented pitcher he had ever seen.

“Jake’s got the stuff, and he’s got the mind-set,” Hunter said. “He really thinks he’s as good as he is. To have the confidence and the ability at the same time is something that not a lot of people have — or ever find.”

For a while, though, Arrieta was just another pitcher with a dazzling repertory who had never made his mark. Trying to throw on a straight line to the plate, as an Oriole, he simply could not command his fastball. The biting curveball, the divebombing slider that he can manipulate into a cutter — none of it mattered.

“I was always in hitter’s counts,” he said. “So all those other pitches weren’t successful because of so many other factors. In trying to take away my crossfire, it changed the way I pitched, and it made me struggle for a long time.”

Last season, after missing a month with shoulder soreness, Arrieta was 10-5 with a 2.53 E.R.A. He took no-hitters into the seventh inning three times and averaged better than a strikeout per inning.

This year has been even better, with 220 strikeouts in 216 innings, a no-hitter at Dodger Stadium on Aug. 30 and a franchise-record 18 consecutive quality starts. Spotting his fastball? You could say he has mastered it.

Joe Maddon, the Cubs’ manager, called that Arrieta’s most impressive trait. It was the same for his pitchers at Tampa Bay, Maddon said, when David Price, Alex Cobb and Chris Archer could not use their curveballs, changeups and sliders until they mastered their fastballs.