Tim Lincecum’s seeming return to form had been good news for the Giants’ rotation, but then Lincecum regressed, giving up five runs in five innings to Arizona on Wednesday night, though the Giants rallied to win the game. So as the Giants head into an important series against the Dodgers at AT&T Park this weekend, their only reliable starter is Madison Bumgarner. Matt Cain was winless in April, giving up home runs in bunches. Ryan Vogelsong has been knocked around consistently. Barry Zito seemed in good form early, but he has had two outings where he’s been knocked out early, probably a sign that he’s returning to his earlier Giants pattern. Cain should return to form; his problem has been losing sharp control, so pitches have been too hittable. That’s probably a matter of mechanics, which can be corrected because there is nothing unusual about his delivery. Lincecum remains a puzzle because he had looked like the old “Freak” for a time before reverting. The league has probably caught up to Vogelsong, and I have no confidence that Zito will stay focused.