Nobody said this rookie stuff would be easy. As first-round pick Marquis Teague struggled mightily, the Bulls dropped their summer-league opener 79-74 to the Celtics on Tuesday night at Cox Pavilion. Teague missed 13 of 17 shots and committed six turnovers with one assist in 31 minutes, 30 seconds. “I had a rough game,” Teague said. “I’m ready to bounce back Wednesday.” The Bulls surrendered a 21-2 run to close the first half and trailed by as many as 23. Last year’s first-round pick Jimmy Butler, who finished with a game-high 25 points, led a second-half comeback that pulled the Bulls within five.