What is a good triceps exercise i can do with a rotator cuff injury?

Up to your PT/Ortho: If you're under care for a rotator cuff injury, you really should let your orthopedist or physical therapist work with you on exercises. They will know what you're ready and strong enough for.

Any: The triceps extends the elbow. You don't necessarily need your rotator cuff for elbow extension. You can effectively isolate your triceps with your arms by the side with triceps push-downs using cable. Basically, you probably safely do whatever is comfortable.

Tricep : Tricep kickbacks should be safe for you to do with a rotator cuff injury. These can be done standing or using a bench for support in a bent over position. Keep upper arm by the side with elbow close to the body. Extend the weight straight back. No involvement of the shoulder. Cable pulldowns ok too but just be careful when reaching overhead to get the cable when first start and when finish.

Isolated tricep pull: Down. Using a cable machine, you should be able to position it at shoulder height. Keep arm at your side and gently pull down. Hi rep low weight too start. Thanks.