told me that I should be able to come to work when I was able to play World of Warcraft

From your name I assume you are German. In that case your employer could suck it, even if you played on Monday. You can do whatever the fuck you want if it doesn’t conflict with you getting better. So if the doctor advises to stay in bed and sleep and you run around town for fun you could rightfully get in trouble. But if you have a broken leg you might not be fit for (your particular) work but could of course still play computer games. If you are on sick leave for mental problems you could even go on vacation because that might even benefit your mental health.