SAN FRANCISCO — When discussing the lineup’s struggles on Monday, Giants president of baseball operations Buster Posey referred to the “big three” and the need to get them going. With the lineup, that’s Matt Chapman, Willy Adames and Rafael Devers. On the pitching side, there has been one star who consistently has been at the top of the rotation in recent years, and right now, he’s also struggling. Logan Webb gave up five runs in the fourth inning on Tuesday and didn’t come out for the fifth. The Giants lost 10-5 to the San Diego Padres, falling to 2-6 in Webb’s starts this season. It’s a baffling stretch, even for a pitcher who has had some rough months in his best years, but perhaps there was a reason for some of Tuesday’s issues. Manager Tony Vitello said after the game that Webb was dealing with knee discomfort, which was why Vitello pulled him after the fourth at the urging of the training staff. Webb generally soaks up innings even on his off nights, but he threw just 62 pitches, his lowest in a game in nearly three years. The first three relievers to enter gave up runs, killing any hopes of...