Fellani sustained the injury in Everton's 2-2 draw with Wigan last weekend, but the muscle tear was discovered while the midfielder was on international duty with Belgium.
So far this season, Fellaini has been instrumental in the success of Everton, who currently lie 4th in the Premier League table.
With Fellaini definitely ruled out for the next League game at Queens Park Rangers on October 21, the Everton bubble could be about to burst.
Fellaini will join fellow midfielder Darron Gibson on the sidelines as the Toffees will have to wait and see if the Irish international can recover from his thigh injury before the end of the international break.
If Everton are without both Gibson and Fellaini, the next few games become all the more important for the Blues.
Thankfully, QPR have been simply woeful so far this season and even without Fellaini, the Blues should be able to see of the challenge of Mark Hughes' side.
Phil Neville and Leon Osman are a big step down from the Gibson-Fellaini partnership, as neither can offer Everton what the injured pair can—Gibson's ball retention and his ability to dictate play along with Fellaini's versatility and attacking threat will be sorely missed.
With Liverpool on the horizon, Evertonians should be a little concerned.
Liverpool have been poor so far this season, but every Everton fan wants the strongest side for the Derby.
Fellaini has been a thorn in the side of each opposing defensive line this season, and while he has had an off game here and there, he has been largely excellent.
Without his aerial prowess, strength and control in the middle of the park, the Merseyside Derby will be an even more interesting encounter.
Of course, Fellaini may recover in time to face the foes from across the park, but Everton will have to game-plan without him.
With striker Nikica Jelavić in great form, the left-side of Leighton Baines and Steven Pienaar have been brilliant and with Kevin Mirallas emerging as a right-sided threat, Everton have the tools to beat a faltering Liverpool side.
Another worry for Everton fans is Fellaini's injury history.
The Belgian international has had two of his five seasons at Everton ended through injury, and a knock this early in the campaign is not good news.
While this knee injury is not too serious, the next one could be and Everton manager David Moyes would want to see Fellaini continue his recent run of excellent form, rather than nursing a muscle tear.
Thankfully, the injury has been picked up during the international break, so Fellaini—at the moment—is only scheduled to miss one game, and it is a game Everton should win.
But Evertonians will be waiting, hoping, praying that Fellaini can recover for the Merseyside Derby and beyond so the Blues can build on their strong start and push for European football.
The Blue bubble has not burst yet, but it is in a precarious position.