So, what has Rossi done in one season if he still needs more time to learn the character of his players as Roma? As a coach, you must comprehend it as soon as possible so that you may choose the best plan for your squad. I believe you stated incorrectly that Rossi needs more time to study the peculiarities of his players.
I believe Rossi has succeeded, although Roma now lacks a large number of quality players. Roma's defence looks terrible.

Look at Roma's statistics from last season. It has been proved that Roma was performing poorly. The disparity between GF and GA was not large. It suggests Roma is strong on offense but very weak on defense.
I see a change in the game under the direction of De Rossi, previously AS Roma played negative football and only defended a lot under the direction of Jose Mourinho, but have they become strong and solid when defending? I don't see that, whereas when De Rossi was appointed to replace Jose Mourinho, he tried to change the game usually shown by AS Roma, under De Rossi's direction they were more able to attack and I think the results were quite good too, even though it still left gaps in back line.
He has many opportunities now to improve the squad he has and improve positions that are still weak. If AS Roma is willing to spend more money to recruit players, I think they will be an even better team next season. because currently there are developments that I can see from the strategy implemented by De ROssi. But I also can't guarantee that they will be better, only that there is hope.