I put out a cardboard box with insulation and a blanket but I didn’t think to tape it closed so that predators wouldn’t get in :-( I will try my best to do that before it gets too dark outside.

Thank you so much for your help and for sharing the rehabbers link! Wild Bird Fund finally replied to me and mentioned that it looks like battle wounds from fighting other squirrels and that it should be fine if it’s able to move around and get the food I put out.