If you've ever had a migraine or cluster headache you'd understand that they are serious medical issues. They can effect your vision, balance, and coordination in addition to just being plain painful. Not something you can play through if the symptoms are severe.
Also they often get worse if you're active and moving around exercising. Accelerated heart beat, light, noise, and motion all aggravate the condition.
Now that's not to say he just wasn't hung over and lying about it to get out of practice. Migraines are the go to excuse for many drug seekers just because they're pretty much impossible to conclusively verify outside of an immediate brain scan while they're occurring.