He has been average since December, and actually shit for the last two months. It isn't that after this mistake, I went from he's amazing, to he's good but not great.
He good do no wrong on eyes of many. He leaves his position and we concede a goal in the progress, 'it is not his fault cause he is surrounded by shit'. He leaves his position to go at the middle of the pitch to win a header which was completely un-risky and in the process drops both himself and Blind on their arses, conceding a goal 'it is not problem cause Blind cannot win a header'. He can't make a pass to save his life 'meh, defenders are there just to defend'. He has to cover a single player (Carroll) from the corner when he loses the header and we concede 'again it is someone's else fault'. He makes a complete stupid penalty 'every player does the same', etc etc. There were actually comments in this thread that he had Lukaku on his pocket, when Blind was dealing for the entire match with Lukaku.
The list of the excuses for him now is longer than that of Welbeck who apparently played somewhere between a defensive right back and a keeper.
For what is worth, I think that Smalling is a good player. I wasn't deluded enough to proclaim him as the best in the league after 2 good matches, and neither can say that he had a fantastic season when he was average for more than half of it. He had a decent season, the best he has ever had, when on 3-4 months he might have been the best performing CB in the league, and on the other months he was hit and miss, with the majority of time during those months outshined by Blind who is nothing special. He has been a top 3 - top 5 player for us this season (below De Gea and Martial, around the same level as Blind and probably Rashford). I think that both our CBs are good but not great, and ideally they should have been bench warmers, or at least partnered by someone who is much better than them. Signing a CB should be a priority this summer, and then depending on his strengths and weaknesses, one of Smalling/Blind can hopefully complement it (i.e if he is small like Marquinhos, then Smalling would be better, if he is shit at passing but he's strong and tall, then probably Blind would be better).