Rookie point guard Marquis Teague is doing the only thing the Bulls want him to do in the summer league this week. He’s learning. Teague, the 29th overall pick of the NBA draft who won the 2012 NCAA title with Kentucky, was selected after Derrick Rose tore knee ligaments in the playoffs. But he’s not expected to be Rose’s replacement. In fact, he’s not expected to be anything but better today than he was yesterday. ‘‘He’s 19 years old,’’ Bulls general manager Gar Forman said as he watched Teague play markedly better in his second summer-league game than he did in his first. ‘‘We didn’t draft him for need. We drafted who we thought was the best player left on the board.