You can refuse to sign any unfair discipline or performance document, noting that it's the consequence of your Workers Compensation claim. You can also consult a Workers Comp lawyer in your area. This is especially prudent if the stakes are high for you, such as demotion or job loss, on top of already being injured!
While there is no U.S. federal law prohibiting retaliation against Workers Compensation filers, most states do forbid it. Workers Comp varies by state so check your state law.
Generally, however, one must meet these four criteria to demonstrate retaliation:
1) be a covered employee entitled to Workers Comp benefits -- rather than, say, an independent contractor, another company's employee, etc.;
2) show that you engaged in some protected activity regarding Workers Comp such as filing a claim or a workplace injury report;
3) show that you suffered an adverse employment action as a result of your filing the Workers Comp claim, such as being discharged, demoted, formally disciplined, having your pay decreased, etc.; and
4) demonstrate that this adverse action was motivated by your Workers Comp filing or other covered Workers Comp activity.
Note that the reason why employers might be motivated to retaliate against Workers Comp claimants is to keep their premiums low and to discourage other employees from filing claims. There's a lot of pressure on some companies to handle workplace injuries through one's private insurance instead of the Workers Compensation system.
On the other hand, please beware that companies do use performance reviews and violations of company policy as successful defenses against retaliation claims. What that means is that if you were in ANY way responsible for your injury (e.g., not wearing personal protective equipment, engaging in horseplay, not following the lockout/tag out procedure, etc.), then the company would probably use this to justify its action.
The bottom line is that you know what happened with your workplace accident and whether you bear any legitimate responsibility. Let that guide you.