Oh my goodness, I am sorry about this poor little mouse. 
I know there are others on the forum who are much more experienced in dealings with injured wild mice/rats who can advise you on this, but at least I wanted to say that it is so nice that you care about this little one's welfare and I do not blame you for not wanting to set him/her out into the wild while it is injured.
Do you have a local wildlife rehabiliation number or website you can look up to get some solid advice on what you should do that is best for the mouse?
I hope all goes well for this little guy.
I know there are others on the forum who are much more experienced in dealings with injured wild mice/rats who can advise you on this, but at least I wanted to say that it is so nice that you care about this little one's welfare and I do not blame you for not wanting to set him/her out into the wild while it is injured.
Do you have a local wildlife rehabiliation number or website you can look up to get some solid advice on what you should do that is best for the mouse?
I hope all goes well for this little guy.