The news was confirmed by the Belgian FA last night, meaning he will miss the country's World Cup qualifiers, and likely Everton's next match away to QPR.
It also means the Belgian is a possible doubt for the Merseyside Derby at Goodison Park on October 28.
So is the timing good or bad? Well an international break means the midfielder has extra time off to recuperate, and perhaps a chink of light is that at least he can't pick up a more serious injury while playing for Belgium.
Here is a look at how Everton fans have reacted to news of the injury...
@adhunter1984: Fellaini injured. Noooooooooooo.
@daveybilo: Fellaini out injured for three weeks... Absolutely gutted. Gonna miss the derby
@sionthomas: I'm well gutted that Fellaini is out for 3weeks! Hope he's back to play vs Liverpool
@davidandalbert: All Fellaini's injury means is that Suarez is going to have to try and get somebody else sent off
@evertoniandy: Fellaini out for 3 weeks and missing the derby? I really hope not...
@scott_morrisefc: Genuinely ****ting myself for the derby knowing Fellaini might now be touch and go. This is going to be a worrying build up.
@joker1878: Was inevitable that Fellaini or one of our top players would be out for the Derby. Predictable !
@evertonnewsfc9: Going to have nightmare's about Fellaini's injury tonight, sleepless night
@johnnymcplease: Of course Fellaini was going to get injured, things were going well
@karlhughes5: Fellaini could possibly miss the Derby? Hopefully Gibson's 100% fit and Little osman stays clear of Injuries! Be a big loss!! But osman laaa
@kayne_efc: In my opinion play Fellaini injured instead of Neville in midfield for the derby, okay
@adzdj: The real genius tho, is pretending Fellaini is injured for the derby
Are you concerned or are you confident Fellaini will recover in time?
image: © nicksarebi