Matz experienced soreness following Sunday's rehab start and will receive a second opinion on his lower-back strain in the next few days, Lynn Worthy of the St. Louis Post-Dispatch reports.
The left-hander initially suffered the injury during his start in Detroit on April 30. Prior to the aggravation, he was closing in on his return to the Cardinals, as Sunday was his third rehab start. It's unclear how much longer Matz will be sidelined, but there should be more clarity once he's further evaluated.