Well, I'm glad that some of his mistakes and weaknesses are finally being looked at instead of all the overhype that there has been about him in the past. However, in the interest of trying to be objective and consistant, I personally wouldnt say he was completely at fault for the first goal. I would say it was both Bailly and Smalling who as a combination could have done more.
Smalling's action of planting himself like he was going to be able to intercept the pass, left the 1 vs 1 between Bailly and Ben Yedder. On another day perhaps Smalling stops the pass but honestly he didn't look that close and he didnt manage to do much to make the pass more difficult for Sevilla.
So Smalling was unsuccessful and unfortunately that meant he couldnt do much to get back and help afterwards. Normally this would be okay as you'd back a good CB to deal with a striker 1 vs 1 and make sure he doesnt get a clean, good strike at goal. However Bailly wasnt up to the task at all and Ben Yedder's touch and shot were good and his mistake was exploited there.
So yeah for me its a mistake by both Smalling and Bailly, together. One of them should have come good whether it was Smalling making the pass too difficult or intercepting it, or being in a position to get back and help Bailly afterwards. Or if it was Bailly who was comfortable 1 vs 1 and managed to restrict the danger. One of them should have come good and neither did.