Well, this is all sorts of cheeky and very funny but we're not sure Mourinho will be impressed.
Mourinho is under all sorts of pressure after defending Premier League champions, Chelsea, suffered their worst start to a league season since 1990.
That's not great, is it? And this week seemed to have tipped Mourinho over the edge, after defeats to West Ham, Stoke City and a 3-1 thumping by Liverpool.
With Chelsea falling to another loss, there were chants in the stadium by the Liverpool fans that Jose would be getting sacked in the morning.
And pretty much everyone on social media reckoned he was in for the chop - although we're not sure his players reckon he'll be going soon.
Then again, that didn't stop poor Costa falling victim of a very funny and pretty cruel prank. Yep, one football fan managed to get him to sign Mourinho's P45!
Costa wasn't the only one who managed to get pranked, though. As you can see, Mark Lawrenson and Manchester United's Juan Mata both fell foul of the cheeky trick.