While celebrating Chris Johnson being picked by the Miami Dolphins in the first round of the NFL Draft, the  San Diego State Aztecs know they have some work ahead of them in replacing him. It goes beyond the loss of Johnson, who spent his entire four-year career at SDSU. Fellow cornerback Bryce Phillips also used up his eligibility and hoped to be drafted in the later rounds or sign with a team as a free agent.  In fact, the secondary, which was one of the big reasons for SDSU’s ballhawking defensive identity last year, will have to replace four of five starters.  Johnson set the bar high with one of the best seasons by a defender in Aztecs history. He was named the Mountain West’s co-Defensive Player of the Year after piling up 49 tackles (36 solo), four interceptions for 146 yards and two touchdowns, nine pass breakups, three tackles for loss, a sack, a forced fumble and two quarterback hurries.…