Nicolas Otamendi, a player who looked on his way out of the club after barely featuring last season — just as the fixtures start piling up this season.Guardiola’s response? Bring it on.“It’s not a problem, it is a challenge,” he said Friday about City’s center-back crisis. “What’s important is being positive in your mind.”Guardiola, therefore, is going to have to be bold and creative. His first decision was to play defensive midfielder Fernandinho at the heart of the defense for the Champions League group match away to Shakhtar Donetsk on Wednesday. Making his first start of the campaign, Fernandinho — who has been playing as a defender in training this season — slotted in just fine and City kept a clean sheet, winning 3-0.Expecting Fernandinho, 34, and Otamendi, 31, to play three games in a week-long span might be asking too much, so Guardiola will have other ideas.Like, maybe, promoting Eric Garcia to the first team. The 18-year-old Spaniard was part of Barcelona’s famed La Masia academy as a kid and was signed by City in 2017, but has yet to play a Premier League match despite being in the senior squad all last season.Or even Taylor Harwood-Bellis, a 17-year-old…