The way I see it, Moyes has clearly changed the system since 2nd half of last season, pushing Fellaini further up to replace the attacking mid berth vacated by Tiny. That's clearly not in dispute.
On the surface this seems like a good idea - Felli's proving it by bagging every game he plays lately. However, Fellaini presents more of a target for all manner of passes than Tim ever did, in no small part because he can beast people both in the air and on the deck. When Tim was around, by contrast, our midfield would look to play the ball straight to Jela who'd made the early run towards the 18 yard box. Hence why Tim seemed useless last season. Back to Jela though, this is what he seems to thrive off - playing off the shoulder and making clever runs, along with balls whipped in low to the 6 yard box in the event the other team's defence has got itself back in position in time. At this stage, Tim would have taken up a position on/near the penalty spot and lurk in the hope of a cross or a rebound, which rarely came as Jela had already buried it.
With Felli now fulfilling this role though, Jela finds himself largely redundant as the ball always comes in to Fellaini who then, 9 times out of 10, passes it out wide so he can get in the box to get on the end of the subsequent cross, or has a go himself. What role does that leave for Jela? He's not likely to be the first choice target for any cross when Felli's in the vicinity, so all he can really do is try and find space to present a clear option for any follow up attack, or use his movement to stretch the defensive line. With a lack of pitch to push forward into, he ends up at the periphery of the box/the back post. When we seem to want to play it intricately through the final third he ends up finding himself as an option for the wingers/full backs and therefore far from being in a goal scoring position. For someone with his finishing ability, this is criminally inefficient use of our primary goalscoring threat.
I'd propose dropping Felli back in second halves, especially if we're already leading at the interval. Get him to beef up the central midfield with Gibson, allowing Osman to get forward and be more creative in the middle, therein giving Jela a good stab at getting on the end of balls intended solely for him to do his sweet sweet thang with, as per last season. Felli could still get up for set pieces with Ossie filling in for him, so it's not like we'd lose his aerial threat entirely. Would also mean he'd leave some in the tank for if/when we really needed it.
Current system is simply squeezing out our loveable Slavic wonder, and I don't think there's much he can do about it until we start playing ever so slightly more directly again. If not, then maybe give Vellios a couple of starts after the Spurs game and see if he can do any better. Jela on the bench can then come on absolutely ravenous for an opportunity to slot, and sh!t right up jarg opposition defences who have already been tenderised by the big boys.