The credibility of the police is usually not the main basis of assessing evidence. It's not really that often that the authenticity of evidence is called into question at all. But sure, it could happend.
We are talking about them breaking the law. The reason there credibility usually isn't questioned is because they know that if they do that the evidence will be rejected so they are compelled to be honest. Once that is out why wouldn't they be questioned they are proving a willingness to do something illegal to get a conviction. Manufacturing evidence isn't unbelievable at that point.