I've had two injuries since I started practicing yoga, an old knee problem that flared up while walking too long in London and the sore coccyx I have at the moment. My response to the knee problem was to only practice asana on one side, with the coccyx I'm a little better prepared.
This is where Vinyasa Krama comes into it's own. Having trouble practicing the seated* Marichiyasanas ( and I am), that's OK, there's a standing option. In fact, for pretty much all the seated postures in Ashtanga there are standing versions in VK, whether on one leg or two. Getting fed up of Standing postures? That's fine, there are versions of the seated asanas in Supine, in Shoulderstand and even headstand.
If you've came to Ashtanga from other styles of yoga then you've probably been exposed to different asanas and possibly practiced a different sequence every time you took a class. I started out with Ashtanga and it's fixed sequence, it was the only yoga I knew, I was terrified of wrist injuries.
Ramaswami recommends familiarity with as wide a range of asana as possible. Practicing the different asana allows you to access all areas of the body with a high degree of localization but it also gives you a wide range of options when faced with injury, whether your own or, I would imagine, your students.
One of the things I noted while reading Mohan's, Krishnamacharya and the Guruji book was how both teachers we're able to respond to students, referred to them by doctors, with individualized practices. I remember too, Manju Jois mentioning, in an interview how, when he would struggle with a posture, his father would give him asana that would help prepare him for the more difficult posture.
*Seated postures are OK for me at the moment as long as I sit far forward on my sit bones. Still able to do my long Paschimottanasanas and my lotus is fine for pranayama and meditation. Actually most postures are OK, it's more a case of getting in and out of them that's painful, in ashtanga I've always rocked back a little before lifting up, perhaps it's time to break that habit and work on lifting straight up.