I remember over the summer I was saying on another site that we should sell Glenjo as his value will likely go down in the future before rising, and his form yo-yo's too much for him to be reliable, and got a ton of criticism for it! Something about us not being a selling club, Glenjo being a world class fullback, etc.
(These were the same people that claimed that Agger was world class, Skrtel was a fast ball carrying CB, and Lucas the best Defensive MF in the league, mind. They eventually banned me for pointing out that the paid bloggers couldn't bother spellchecking their articles! Gonazlo Higuain was a transfer target, and Liverpoo FC was on the rise evidently...they were pretty egregious errors.)
Anyways, this has always been my concern with Glenjo. Average at best defensively, quite decent going forward, but the consistency has never quite been there. I'm sure his form will come back, and he'll be a decent option before the season ends, but I think it's time to start looking for a more consistent option. It definitely feels like the academy produces players who are at least fundamentally sound, even if the flair isn't quite there, and I would say Flanno is a good example of that. He might not be amazing at any individual aspect, but he gets forward at the right moments, does well enough defending(at LB too), and has enough ability on the ball(something Cissokho lacks) to at least be a viable option. For this, I think a fairly average Kelly would be a solid option at fullback.
It feels like a lot of our issues come from trying too hard to play the style of game more than the game itself, and having more fundamentally trained players might help us, especially at fullback. It's not a vital enough position to where a lack of true flair and ability will be a hindrance, but an important enough position to where consistency is very important. If you're going to spend 10-15M on a player, I would rather it be on a more active position, as I feel fullbacks are the least vital of the lot. You have to have good midfielders or you get run through, you must have good centerbacks or your back line gets shredded, having good strikers/wingers is a requisite for goal threat, and a top keeper can bail you out big time. Fullbacks just need to be good enough, and while having top options is a huge boost, simply having bodies that don't screw up is way better than having form players.
tl;dr: A bad fullback hurts you a lot more than a good fullback helps you.