Roma coach Jose Mourinho said the challenge that awaits him this season is different to many of his previous jobs, insisting it will take time for him to leave his mark on Serie A.

Mourinho, who steered Inter Milan to an unprecedented treble in 2010 during his last spell as a coach in Italy, has returned to Serie A to take over from Paulo Fonseca at Roma, after they finished a disappointing seventh last season.

"Roma is a giant club, the size of many other clubs I have been to," Mourinho said ahead of Sunday's league opener against Fiorentina.

"The nature of this project is another thing. When I arrived at Chelsea, Inter or Real Madrid, the projects were obvious - to win from day one.

"Roma are in a different situation. They are a club that hasn't won in a long time and that last year finished 29 points from first and 16 from fourth.

"It is obvious that it takes time and in football time is important. The reality is that we have not won for many years.

"There is a lot of work to do but the results will come."

Roma dipped into the transfer market this week to bring in striker Tammy Abraham from Chelsea in a €40m deal, replacing outgoing veteran forward Edin Dzeko.

Mourinho, however, wants to see more signings before the summer transfer window shuts on 31 August.

"I already explained our transfer strategy was one of reaction, because we lost two players that we weren't expecting to lose – Leonardo Spinazzola (through injury) and Edin Dzeko," Mourinho said.

"We had to react and didn't therefore sign the players I thought would've been important to balance the squad.

"I don't have the right to pressure the club or make more demands. I have to work with the players in the squad, but we've still got time to do something more in the market."