If you're injured and can't run, you can maintain some running fitness by substituting elliptical training or water running.  (But if you're injured, please consult your doctor or physiotherapist before substituting any run workouts.)

ELLIPTICAL MACHINE: You can use the same HR zones you use for running.

WATER RUNNING: Because of the hydrostatic pressure of the water, don't worry about your HR at all and just use the rate of perceived exertion, as per what's dictated in your prescribed workout in the app.

With both options, you can maintain the number of runs you do each week but shorten the prescribed duration to between 50-75%, making sure you're not pushing any more than is comfortable.