Justin Verlander underwent successful core muscle repair surgery on Thursday, the Tigers announced. He suffered the injury during an offseason workout last month. The team says he will rehab for the next six weeks and then be re-evaluated. “We fully anticipate Justin to participate in spring training and be in a position to compete at the beginning of the 2014 season," said GM Dave Dombrowski in a statement. The nature and extent of the injury are unclear. Verlander's run of six consecutive opening day starts could be in jeopardy if he is limited or slowed in camp at all. That is the second longest such streak in franchise history, behind Jack Morris' run of 11 straight opening day starts from 1980-90. Verlander, 31 next month, went 13-12 with a 3.46 ERA (121 ERA+) in 34 starts and 218 1/3 innings last season. He is baseball's preeminent workhorse, throwing at least 201 innings every year since 2007. The Tigers did scale back his workload a bit in 2013 in an effort to keep him fresh for the postseason.