SAN FRANCISCO — A year ago at this time, Hayden Birdsong looked like a future frontline starter or closer for the Giants. But the young right-hander now will need to wait until 2027 to get back on a big league mound. The Giants announced Thursday that Birdsong will undergo Tommy John surgery next week in Dallas. The 24-year-old felt discomfort in his arm on the second-to-last pitch of a relief appearance on March 10 and an MRI revealed a Grade 2 strain of his forearm and a UCL sprain. The procedure will be done by Dr. Keith Meister, who was consulted this week as the Giants went over Birdsong’s options. Birdsong said over the weekend that he felt an odd sensation, similar to a rip, when he threw a curveball in his final spring outing. He called it a “freak” thing that just happened on one pitch. He did not deal with arm issues last year or earlier this spring. “Obviously I’m not happy about it, but it’s one of those things that I feel like everybody kind of goes through at this point,” Birdsong said on Sunday. “I wish it wasn’t right now. I wish I could throw a couple of seasons before something...