Believing you are ready to go and able to help your team and doing so are two different things.
Athletes play hurt, and are allowed to do so, because it's hard to tell how they are going to do. There have been some amazing performances by hurt athletes who were toughing it out and some lousy ones. Colombo turned in a lousy one.
The Cowboys played him because his rehab from injury was completed, and because they believed a healthy Colombo was their best alternative, as I'm sure he did. I don't think any athlete plays (well, there might be some) if he thinks his performance might hurt his team.