This has happened with two or three of our chickens. We know one was definitely due to injury, two birds got in a sort of squabble and one fell, or something happened to her foot, but after a week or so she was much better on it, though she's still limping a tiny bit 3 weeks later. The other couple limping birds got better over time. Just make sure they can be a quiet, nonstressful place. A crowded coop is probably not the best area. We also gave ours plenty of hay in the corner she preferred, near the water and food. Some vitamins probably wouldn't go amiss.
With Marek's it's less likely that the bird will start walking normally again, or so I've heard, which leads me to believe (hope) my few limpers haven't had Marek's, since they've all recovered sufficiently within a week or so.
I hope your bird gets better soon!!
Edited to include that when our birds limped they mostly stayed laying down and didn't try to move too much, though they didn't exhibit any obvious signs of pain when we would move the foot around to see if it were broken. If the roo is trying to walk on it anyways that could be something other than injury, unless it's just a roo's nature to not be inclined to rest?