Although Johan Santana came out of it in one piece, manager Terry Collins wasn’t thrilled with the impromptu bullpen session the lefty threw Sunday, and Collins let him know it yesterday. “I asked him, ‘What did you get on the mound for?’ ” Collins said. “[Santana responded] ‘Because I felt good.’ It was unnecessary. In retrospect, he knows he could have handled it a little bit differently, but he’s fine and he wanted to show everybody he was fine.” Santana threw a long-toss session again yesterday and pitching coach Dan Warthen expects Santana to throw another light bullpen session tomorrow — this time, with permission. “He worked hard out there,” Warthen said. “He came out of it very well.” Collins was concerned Santana might have been inviting trouble — which is what the manager told him. “The last thing I need is have you wake up tomorrow stiff,” Collins said of the conversation. “And take a huge step backwards just to prove to everybody you’re OK. I understand what you’re doing, but once in a while, let stuff slide.” As for Santana, he said: “I’m good and that’s all I’ve got to say right now.” Santana has been displeased that his offseason regimen has been questioned, since he hasn’t been able to make a start yet this spring. Jeremy Hefner, who could slide into a starting spot if Santana begins the season on the disabled list, gave up a solo homer to Justin Upton, but was solid in a 4-2 loss to the Braves. He surrendered just one run in three innings.