CHICAGO — Red Sox rookie right-hander Richard Fitts was cruising through his third start of the season Saturday. Then, suddenly, injury struck in the bottom of the sixth.

After tossing five shutout innings as the Red Sox claimed a 2-0 lead, Fitts unleashed a 2-2 sweeper to Miguel Vargas, then immediately grabbed under his throwing arm in clear discomfort. Manager Alex Cora and a team trainer visited Fitts on the mound and promptly pulled him out of the game. After a prolonged warmup session, reliever Zack Kelly came on to finish the at-bat.

According to the Red Sox, Fitts is dealing with “right shoulder pain.” The exact nature of the ailment is unknown.

Before the injury, Fitts had allowed just two White Sox hits while striking out five. He was pitching with a lead after Ceddanne Rafaela (RBI triple) and Romy Gonzalez (RBI single) plated runs off lefty Martín Pérez in the top of the fifth. Fitts threw 76 pitches (54 strikes) in total.

Fitts has a 3.18 ERA through three starts this season. He entered the year as Boston’s No. 4 starter once it became clear Brayan Bello, Lucas Giolito and Kutter Crawford would begin the year on the injured list. Bello and Giolito are close to making their season debuts and Triple-A righty Hunter Dobbins, who debuted Sunday, will likely be an option for a promotion if Fitts needs time. Cooper Criswell is also in Worcester.