Back to the hip flexor, as Baldkin said, you just never know with it. It can be a very naggy type of injury or feel fine by his next start. In high school, I had a naggy hip flexor. I was able to massage it and stretch it enough so I didn't miss any time in track or baseball, but that obviously meant my issue was minor. I was a lefty pitcher and it was my right hip flexor - if it was bothering me, it was uncomfortable when I planted right foot as I strode towards home plate. I was still able to pitch when it was bothering me, but like I said, it was just a mild injury - if it was serious, there's no way I would've been able to pitch.