De Rossi did his job well but it's not true to say his coach results are better than what Mourinho did for Roma in two previous seasons. He already was given chance to work longer with Roma and he will have many more pressure in a new season after the club decided to spend considerable transfer money in this summer.
This isn't something to be really worked up about, it's always a normal for people to quickly forget so soon how well one did performed immediately they are gone, Mourinho did stayed two seasons at Roma and ofcourse his first season was a beautiful one and the little challenges he had in the second turned out such that got him sacked and Rossi came in and did well but isn't enough ro be compared to Mourinho just yet, they haven't got equal stay time and just like rossi is doing, Mourinho did well too in his first season. Personally I wouldn't be seeing the reason for any of these comparisons, it may not be necessary. Let's allow the coaches perform at their capabilities.
Yes indeed, it seems inappropriate to compare De Rossi with Mourinho because after all, basically Mourinho has had a lot of experience in his coaching career and it is different from De Rossi who can still be said to be new in his coaching career. But even so, De Rossi as an AS Roma legend is also able to provide good results for AS Roma because with De Rossi's success last season also made AS Roma manage to finish in sixth position and get a Europa League ticket.
Thus, maybe that also makes AS Roma quite active in this transfer window by buying several players. Of course, with some of the new players brought in, it will be a hope to be able to make a good contribution to AS Roma next season. Because AS Roma who will also have a schedule on, ofcourse, will need a rotation of players who thus also, AS Roma need second-tier players who at least also have good quality to keep the team's performance good.