That, Whirly, can apply to virtually every player on the pitch though. CBs, FBs, GK, DM who mess up and it leads to a goal, MF and attackers who fail to take their opportunities (e.g Suarez and Sturridge at Arsenal) and it changes the course of the match or the result.
You have to look at the balance of outstanding/good/poor matches throughout the season. I think for most people Johnson would fall in the top echelon of performers at the club and is therefore worth his pennies.
Small sample size or not, if you compare Glenjo and Enrique, I would take Enrique every time because he checks more of the boxes that I look for in a fullback. For me, your fullbacks just need to be capable players, not world beaters to be a good side. Everybody likes a sexy attacking fullback, but if you look at a player like Enrique, he's rarely responsible for us conceding when compared to our RBs, but still manages to do enough in the final third. There's certainly more to their play than the goals and assists figures, but where Johnson uses his agility to get into attacking positions and put in a cross or cut in and shoot, Enrique typically links up with Suarez/Coutinho. You get roughly the same end product, but I think Enrique fits our play style much better considering crosses aren't our preferred way of scoring, and we have many other players who should be shooting before our fullbacks are.
If you add in that Glenjo is one of our top paid players, on a wage close to as much as double as what Enrique costs, and is also so injury prone, it's a fairly skewed setup. Glenjo is awesome when he's on his day, and is worth every penny, but it definitely feels about 2/3 of the time that he's either nowhere to be found, or seriously lacking in defense. If you look at it on the balance as you say, he's very average, but as a player he's definitely well above average. And as you said, strikers failing to take their chances, and so on also need to be looked at on on a holistic level, but if you're going to have a fullback who contributes directly to say 5 goals an year, and then directly leads to us conceding just as many, which is about the kind of player Johnson is, I don't know if that's the ideal setup, especially when compared to a player like Enrique who plays a role in the same number of goals, but absolutely leads to us conceding fewer.
It doesn't really help that he gets injured so frequently, leaving us in a situation where we have to play youth players, or a CB at RB either. It's definitely a trend at this point, which is another concern. If he can play the rest of this season, at this clip, without getting hurt, then he's worth what he's paid on the balance of his injury/form plagued seasons of late, but until then I remain to be convinced.
