This has to do with the filesystem being used, not rsync itself. Assuming ext, look at extundelete. There are similar tools for other filesystems.
There is no guarantee that you will get the files back. The files will remain on disk once deleted, but the act of deleting is really marking the space that they occupy as being available to other files again. This means that other files may overwrite the data that once was mapped to that file. For this reason, you should avoid writing data to the filesystem before trying to perform the undeletion.