Because if they maintain an inconsistent performance in the first half of the season it would be much more difficult to reach their goal in the league. They would surely be expected to finish the season in top four to make a real improvement.
Empoli is currently also in a slump and this should be something that should be well utilized by Roma.
They are still quite good and can create goals, it's just that until now Roma are still bad at defense and Mou must now be able to improve himself and performance. Lukaku should also be better prepared now than when he debuted as he has had good time to rest and create chemistry at Roma.
Lukaku's contribution can play an important role this season. While Abraham is injured Lukaku should make use of this time period by scoring as many goals as possible. He is back in Italy again even though the team he is at isn't Inter. I don't expect him to perform incredibly well of course. But he can be quite beneficial with his goal or assist contribution.
Roma would really make use of a big improvement in the attacking line. Their defense line isn't in a great shape for now and they can make up for that by being more productive.
Like you rightly suggested, I think he'll have a good spell at Roma under the guidance of Jose Mourinho who knows how to efficiently manage strikers of f Lukaku's age and style of football. In his second spell as a Chelsea manager, Jose Mourinho brought back Didier Drogba back to the Stamford Bridge and brought out the best from the Ivorian as his contributions helped Chelsea win the 2015 English Premier League title. Just like he did with Drogba, Mourinho will Lukaku return back to his best