Yet with every step they make on the pitch, the pair become increasingly vulnerable to the prying eyes of opposing clubs, especially given the Toffees' fragile financial situation.
For Fellaini the situation is simple. The more he replicates his devastating displays of so many games this season, the more potential suitors will covet his signature.
Whilst his contributions are of course imperative to Everton's current ambitions, how will such an impoverished club be able to willingly fend off bids approaching the £30 million mark?
At least with Fellaini, any departure will leave the club excessively well compensated and hopefully able to rebuild. With Moyes, the situation is a little more precarious; Everton's manager has just over six months to run on his current deal.
His most recent quotes hinted at some February talks, presumably so he can judge how the club navigate the January transfer window.
A few signings and no major sales, and he would surely commit. Yet if Fellaini or Leighton Baines suddenly jet off to pastures new, Everton's landscape could become dramatically altered.
Come 2013, Moyes will be as cheap as he's ever likely to be for other clubs.
Added to that, he's suddenly masterminded another Champions League push at the Toffees, despite spending less than he's sold during this calendar year. A rare trend for managers.
Who is more vital to Everton?
His length of service at Everton often makes him an unfairly unfashionable choice for the top clubs, but coming at such a bargain price, and backed by this season's scintillating form, some may well start to reconsider.
Losing either Moyes and Fellaini would be a devastating blow for the Toffees, sure to set them back considerably.
At least Fellaini would leave the club well reimbursed, with a hugely soothed bank balance, hopefully after dragging the side into the Champions League.
However, if the Toffees don't rectify Moyes' contractual situation, they could well lose one of their finest managers for next to nothing; a scenario that must not transpire.