Hi Cyndi! Fingers crossed that the injection works for you!
In this great adventure of maintenance, I've had -- gulp -- nine surgeries and recoveries, including rotator cuff repair and both knees replaced. So I've had to deal those mandatory layoffs from exercise. Just like you, I was panicked at the idea of not being able to exercise, but what I found is that it's true what everyone says: weight loss/maintenance is at least 80% what we eat.
What I did was have a lot of healthy food on hand (I cooked and froze in advance of the surgeries) and stuck to my food plan like glue. It's really easy when you're hurting and in recovery to use food as a "poor me" kind of treat, but that's what has to be avoided! I was scrupulous in sticking to all my weight loss/maintenance skills, like planning meals in advance, weighing and measuring portions, logging every bite, lick and taste, and sticking to my daily calorie limit.
The result, every time, was maintenance. No weight gain.
So I think you're going to be just fine for the 2 - 3 weeks that you can't run. Just stick to your food plan 100% and don't fall into the trap of consoling yourself with food. And if you're allowed to do any exercise at all -- any lifting, maybe? -- that would be a bonus. But if you're careful and practice all your good maintenance skills, I think you're going to come out of this without a gain and maybe even a loss.