Hi and welcome to the forum
If you post a picture of the fish we might be able to identify it and confirm it as a red devil or something else.
If the image is too big for the website, set the camera's resolution to its lowest setting and take some more pictures. The lower resolution will make the images smaller and they should fit on here.
If you can post a short 20 second video of the fish moving, we might be able to see if there has been spinal damage from the attack. You can post the video on YouTube and cope & paste the link here. We can view it at YouTube.
-----------------------
If the fish does have spinal damage it might be able to swim but it would be best to keep it on its own so it doesn't have to compete with other fish.
Do a 75% water change and gravel clean the substrate every day for a couple of weeks. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media in a bucket of tank water.
The daily water changes and filter cleaning will help dilute disease organisms in the water and give the fish more chance to heal without getting sick.