Injuries are for sure a legitimate reason to take ukemi proactively, but the person nursing the booboo is obliged to let his or her partner know of the injury. You don't secretly harbor your damage and expect the partner to understand why you aren't letting them apply their technique or even touch them.

The point of paired practice is to give both persons the opportunity to practice their techniques. Taking preemptive ukemi deprives your partners of their practice. But by informing them in advance of your injury, you can make an agreement to train in a certain way. such as letting your partner go through the motions of the technique and perhaps make light physical contact. The partner gets a chance to use proper form in practice and you avoid aggravating your injury.

If you don't inform your partner, you're leaving him or her with a lack of understanding. Just telling Sensei isn't enough. Sensei and you should be letting your training partners know of your injury and how to go about training to your mutual benefit. If this is something you don't want to do, then you should sit it out until you are healed enough to train again without compromising yourself or your partners. Cooperation requires mutual awareness and understanding for both parties.