Reliever Tom Koehler (right-shoulder strain) threw about 25 pitches off a mound Thursday, his second bullpen session in his rehabilitation from an injury that has sidelined him all season. “He felt encouraged,” Roberts said. “He felt strong.” There is no timetable for his return. … Left fielder Matt Kemp, the team’s best hitter, and shortstop Chris Taylor did not start Thursday. “Matt wants to be in there every day, and that’s great, but understanding and appreciating that it’s a long season, I want him to be at his best,” Roberts said. “We need him for three games in Colorado, so today was good to give Matt a day.” … Ambidextrous reliever Pat Venditte was optioned to triple-A to clear a roster spot for Clayton Kershaw.