I think this season he's been off the boil at times, but there's the huge caveat of a) he's hardly played b) he was fantastic (I thought at least) vs Liverpool and c) he came to the fore with Martinez next to him, who's been injured all season.
When it comes to defenders, we've got Varane, Martinez, Lindelof, Maguire, Evans. On form - which, to repeat, we haven't been on as a team all season - Varane and Martinez are our best defensive partnership without a doubt. Question is whether he can stay fit and if we were to bring in a young top class centre back then I wouldn't be adverse to flogging Varane to Saudi for a large amount of pound sterling.
Looking at the match ratings from his starts to refresh my memory:
Wolves 1-0: 7.0, tied for team-best
Spurs 0-2: 5.5, 2nd best on team
Forest 3-2: 5.6, 8th best on team
Palace 3-0 (CC): 6.8, 6th best on team
Palace 0-1: 5.2, 2nd best on team
Galatasaray 2-3: 3.8, 7th best on team
FC Copenhagen 1-0: 6.2, 3rd best on team
FC Copenhagen 3-4: 3.5, 10th best on team
Bayern 0-1: 5.6, 2nd best on team
Liverpool 0-0: 7.7, team best
Included the second Copenhagen game, where he came on early. Obviously, Caf match ratings aren't exactly a precise science, but they at least give a pretty good pointer towards the general opinion of his performances at the time.
Not really that bad? There are three obvious poor ones, where he got both a low rating and was considered one of the weaker performances on the team: The losses to Galatasaray and FC Copenhagen, and also the win against Forest, where he will have been heavily deducted for his complicity in those two Forest goals within the first four minutes of the match.
The remaining seven, he either gets a good rating, or gets one of the best (or least bad) ratings on the team.
Team record in his starts; 4-1-5, pretty much average.
Goals; 13 scored, 13 allowed= 1.30 pr match. This is just on the season average for goals scored (1.32), but quite a bit lower than the season average for goals allowed (1.56).
So, the long and the short of it - no huge obvious impact on team results, except we allow a little bit fewer goals with him in the lineup. 3 bad performances out of 10 starts, otherwise good, among the best on the team, or both. Hardly the stuff of legend, but in an injury plagued season with a constantly revolving defensive lineup, it's hardly very damning either. Seems to me an awfully thin basis for concluding he needs moving out.
His lack of durability is a point in itself, but I think it's debatable what conclusion to draw from that. As long as he's playing well when healthy, why not keep him around and give the game time to a capable alternative RB option (as of now, Maguire)? Getting a younger, cheaper replacement sounds nice, but the simple fact is a decent replacement is going to come at a bigger cost than what we'll get for him. There aren't dozens of CBs who'll bring what he does. And there are many, many slots in this squad that are in more urgent need of upgrading than his.