It's the defensive side of the game that he needs most work on, he will have a lot of opportunity to show his attacking prowess but how good are his concentration levels? Will he be caught sleeping at set pieces? Is he good enough defending in a 1vs1?
Depends if you see defending as a team job or not.
If you go back and watch Moreno & Can together, when Moreno goes forward, Can doesn't tuck in for him. In fact the best example is the penalty we conceded to Hoffenheim. Moreno closes down the keeper as he is nearest the ball when the move breaks down, this forces a rushed kick from the keeper. You then see Can jog from LB position into his CM position. The ball gets headed into the LB position to the free man who plays in Gnabry behind Lovren. Penalty.
In contrast there were similar moments at the weekend where Robertson bombed ahead of Milner and moves broke down. When the ball got cleared Milner had tucked in as LB for Robertson. One such time was the Benteke chance where Robertson presses his man high up the pitch completely vacating LB. Milner sees this happening and tucks in.
I mention this because we have a habit for criticising players, Moreno in particular, for being out of position. But for Klopp pressure on the ball is far more important. Without pressure on the ball, people can pick out runners with ease, especially against a high line that seems to have problems in reading when to drop (see 3-3 Palace goal, 1-2 Hoffenheim goal, the throughball before the Hoffenheim penalty etc).
Defending is a team responsibility and if someone moves ahead of you, you drop back into their position. There have been multiple times so far when Moreno presses the ball (e.g. Watford 2nd goal, Hoffenheim penalty) that Can leaves a player to run into the space Moreno vacates that he is actually marking.
My biggest criticism of Robertson defensively would be recovery runs. If you watch the Puncheon chance at the weekend for example he is having a light jog to get back. In fairness, he could have been exhausted at that point right before half time.