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.
Well for now we can't compare between the two. De Rossi last season only continued the squad that Mourinho had built. Meanwhile, this season De Rossi is starting everything again for the Roma squad. So it could be said that this season could be the season to prove whether De Rossi is a good coach or not. But to compare with Mourinho then we might have to look at De Rossi also completing the next two seasons. because without it we will not be able to compare them fairly. De Rossi is still young so he might be able to make a surprise to make the Roma squad develop more. And we know Mourinho also had his glory days as a coach. And maybe De Rossi will also start his glory days by becoming Roma coach.