Please don't tell me you judge a player by one match? Funny thing people keep their mouth shut when certain players preform for over a year.
i dont judge players by only one game BUT i dare to say "if you are a good fullback, you wont leave those holes in the back". it is some basic tactic awareness you got which is form independent. i can totally understand when you are not in the best form and you are not able to bypass an opponent while attacking. suddenly forgetting where to be on the field while in defense is a totally other matter to me.
its no one vs. one situation where a better player might get the better of you.
its a situation where you just have to be where you gotta be. no word said about stopping some christiano ronaldo going past you. but if you are not even in the right spot at the right time you have already lost and can not even try to defend.
i really think defensive positioning is not form-dependent. either you have it or you don't.. its a daily routine ... situations that arise in every match played ... you can not forget or unlearn that because you are in bad shape or have no stamina.