Lukaku has actually just lost quite a lot of his playing skills which used to be very good, maybe Lukaku's decreased playing skills due to his aging age because now he is 30 years old.
Inter Milan shouldn't have to regret too much if they fail to have Lukaku because there are still many young players who are more powerful and better than Lukaku.
If Chelsea really let go of Lukaku for Juventus, what Inter Milan should do is look for other suitable player options and can be a good striker at their team.
If you observed Lukaku's performance last season for intermilan you'll understand that he still got it in him even though his skills reduced, because despite his age he still managed to have scored 10 goals in 38 matches for intermilan in the 2022/2023 season and his conversion rate of shots to goal totalled 21, he's still very strong and is seen as a nightmare to most opposition defense and had a total of 46 shots with 19 on target, making him to be rated 7.14 last season
Apart from the Juventus fans kicking against his arrival to their club lukaku might be perfect replacement for Vlahovic at Juventus because he's adapted to Italian league and had a successful seasons scoring a total of 78 goals for intermilan out of 132 match since his arrival on loan from Chelsea, so let's not be too quick to judge him by his age because there are some players who had the best form in their football career at age 30+, however In as much as some Juventus fans feels he's not the man that would help take them back to their glory days, Lukaku is still having talks ongoing with the Juventus boards and sooner or later might move there if both parties finally come to an agreement.