Give this a try.
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^Minecraft-User-Info/player/(.+)$ /Minecraft-User-Info/player/?user=$1 [L]
Edit: Use this since it's in a sub directory.
RewriteEngine On
RewriteBase /Minecraft-User-Info/
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^player/(.+)$ player/?user=$1 [L]
Updated to show my original answer and included the edit per users comment about where htaccess is located.