CHICAGO — Already dealing with a fragile pitching situation, the Chicago Cubs will have to plan around Jameson Taillon, who exited “Sunday Night Baseball” in the second inning with a strained left hamstring.Taillon motioned toward Wrigley Field’s home dugout after walking Matt Chapman, the No. 7 hitter in the San Francisco Giants’ lineup and the only batter he would face that inning. After a brief mound visit with Cubs manager Craig Counsell and an athletic trainer, Taillon walked off the field and Javier Assad jogged in from the bullpen.Foreshadowing this kind of possibility, the Cubs limited Assad to one inning in his last appearance at Triple-A Iowa and promoted him to the majors before Saturday’s game.Taillon, 34, is in the final season of the four-year, $68 million contract he signed as a free agent, part of the group acquired to bring playoff baseball back to Wrigleyville. He set the tone in the wild-card clincher against the San Diego Padres last October, and he successfully handled another elimination game in the next round as the Cubs extended that playoff series against the Milwaukee Brewers.That momentum, however, hasn’t carried over into this season. By allowing one run in his one inning against the Giants, Taillon’s ERA crept up to 5.19. He’s allowed 20 home runs in 13 starts, a sign of his recent lack of execution and shrinking margin for error.Still, Taillon is viewed as a dependable starter, a student of the game and a widely respected player within the clubhouse.The Cubs went into this year viewing the depth of their rotation as a strength. But that depth has substantially eroded since April, when potential ace Cade Horton underwent season-ending surgery on his right elbow.Late May or early June was once the targeted date range for All-Star lefty Justin Steele’s return following his major elbow surgery last year, but a flexor strain has clouded the timeline for his eventual comeback.The Cubs are planning to activate Matthew Boyd during their upcoming road trip this week, though the sharpness of their Opening Day starter is still to be determined.After pitching for Team USA in the World Baseball Classic, Boyd spent time on the injured list to rest a strained left bicep and then recover from knee surgery, a combination of issues that has left him with a 6.00 ERA.Edward Cabrera didn’t immediately elevate the team when he came off the IL, giving up eight runs and failing to finish the fourth inning during Friday’s unsightly 18-3 loss to the Giants.Besides dealing with the blister on his right middle finger, the time off was supposed to help the Cubs reset Cabrera (4.99 ERA, -0.8 WAR, per Baseball Reference) and draw out the stuff they saw when they acquired him in a big offseason trade with the Miami Marlins. Clearly, that is still a work in progress.Chicago’s best pitcher at this moment, Ben Brown, started this season in the bullpen and hasn’t yet completed a full year in the majors, forcing the club to proceed with even more caution.