Waldron suffered a mild left oblique strain while warming up in the bullpen Friday, Kevin Acee of The San Diego Union-Tribune reports.
Waldron's injury prevented him from pitching as scheduled against the Rangers. He'll be re-evaluated Saturday, at which point a return timeline may be established, but he's likely to be unavailable for Opening Day given the timing of the injury. The 28-year-old right-hander has had a tough time getting going this spring, surrendering 11 runs (nine earned) on nine hits and seven walks while striking out seven batters in 9.1 innings.