Defending has a lot to do with fitness - positioning and reading the game are also very important, but a lack of fitness shows. Especially when a high line is played, there are a minimal amount of holding midfielders, and fullbacks (and even centre backs, if your name is Danny Agger) are pushing on all the way up the pitch. The players don't look sharp because they don't look fully match fit - they are unable to recover rapidly when caught out of possession, and are impeded and fatigued by frequent losses of the ball like in the first half.
Johnson was playing his first full match of the preseason, and was obviously caught out on several occasions. Robinson, Flanagan and Kelly will find it tough to get up to the required pace of the game anyway, and are also lacking match fitness. Agger and Carragher missed a few important tackles and interceptions, though that is more to do with concentration and being up to speed mentally than an issue physically.