It’s definitely a racial/ethnic slur.
I think Zlatan was referring to something else:
https://www.theguardian.com/footbal...rhad-moshiri-voodoo-manchester-united-everton
Back when Lukaku and Zlatan were in United, Lukaku was considering taking legal action against Everon's owner over his claims that "Lukaku had a voodoo and he got the message that he needs to go to Chelsea”.
Even Lukaku's lawyer/representative had to deny the voodoo thing: "Romelu’s decision [to leave Everton] had nothing to do with voodoo. He distances himself from these beliefs and this statement and will now see what judicial steps can be taken in relation to them. Romelu is very Catholic and voodoo is not part of his life or his beliefs."
This was happening in January 2018, when Zlatan and Lukaku were playing for United, and I guess it was discussed/joked about in the locker-room. Zlatan being Zlatan, probably knew that he can use this to provoke Lukaku yesterday evening, and he actually did
