I’ve had them in both shoulders for rotator cuff and bicep tendon problems.
Like footflaps and Stoatsbrother said, it’ll take a few days for the joint to settle down from the disruption and the cortisone to take full effect.
After that, bear in mind it’s not a magic healing agent by itself, it’s just a strong anti-inflammatory which stays active in the joint for two or three months. This buys you some time to heal and re-strengthen the tendon progressively while it’s in a less aggravated state. If you don’t do that systematically (ie do your exercises!) you’ll be back at square one when it wears off.