SAN DIEGO—Bob Melvin put on a Padres home pinstriped jersey and stated the obvious about becoming San Diego’s new manager. “What an opportunity. I’m probably as surprised to be here as you all are,” Melvin said Monday after being introduced at a news conference at Petco Park. The hiring of Melvin, who has 18 seasons of big league managerial experience, was a surprise for a few reasons. He was still under contract for one year with the Oakland Athletics, where he spent the last 11 years, and Padres general manager A.J. Preller had a proclivity for hiring skippers with no previous big league managing experience. Melvin inherits a team that had five All-Stars—including Fernando Tatis Jr., Manny Machado and Jake Cronenworth—but underwhelmed after the trade deadline and finished with a losing record for the 10th time in 11 seasons. The Padres were 79-83 and third in the NL West, 28 …