Kevin Alcántara was already working with a limited runway to establish his narrow role on the Chicago Cubs’ roster. And then he made a glaring mistake on “Sunday Night Baseball.”The Cubs have optioned Alcántara, 23, to Triple-A Iowa while planning to activate Matt Shaw from the injured list for Tuesday’s game against the Colorado Rockies at Coors Field, according to sources briefed on the decisions.A healthy Shaw, who had been dealing with back tightness, should provide different options for Cubs manager Craig Counsell as he tries to redirect a struggling team. Over the past month, the Cubs have fallen from 15 games above .500 to 34-32, draining the optimism that surrounded their fast start.Shaw, a Gold Glove finalist as a rookie third baseman last year, was bumped into a super-utility role when the Cubs signed Alex Bregman to a five-year, $175 million contract. Thus far, Bregman’s deep struggles with runners in scoring position have led to repeated boos from the big crowds at Wrigley Field.“I’ve been terrible,” Bregman said after Sunday’s 2-1 loss to the San Francisco Giants in 10 innings. “I need to play better. Offensively, it’s been awful.”One of several missed chances in that frustrating game involved Bregman and Alcántara, a pinch-runner who had raced from first to third on an infield single and a throwing error in the eighth inning. With no outs, Alcántara ran halfway toward home plate when Bregman hit a soft line drive at Giants first baseman Rafael Devers, who threw the ball over to third base for the double play that stopped the rally.