When Captain America saw Bucky, he became emotional and weak to a point where anyone could knock him. But, why did he believe what he saw? It could have been a clone of Bucky (after all young Bucky and he were friends 7 decades ago so alternative ideas seem more natural). Is Captain America fit for any battle when all his enemies need is to wear that face-changer mask we saw in the end of the movie?
When Steve first realizes he's fighting Bucky
When The Winter Soldier turns around and Cap sees him, he is taken aback by the shock. No one will argue that. But he is seeing
- His oldest and closest friend
- A person who is 70-ish years out of time by age
- A person who he not only believed to be dead, but that he personally felt the responsibility of the death
Not only is Bucky alive, but Bucky just tried to kill Cap. This stuns him for a moment. This is a reasonable reaction, even in Marvel Comics. This does not necessarily make him "unfit".
On whether or not his enemies could/should just use face changing masks
Cap goes up against Bucky again, multiple times. In the subsequent battles, he does not stand there shocked. It's true that Cap may be pulling his punches a little bit more than normal, as he doesn't want to injure Bucky badly, but Cap does not typically fight to kill to begin with. The fact that Cap fights him normally later is proof that wearing a face changing mask would not be a universally effective method to defeat Cap.