He scored 28 in 46 matches in total compared to Lukaku's 28 in 35 matches and that's bearing in mind Everton's game was geared totally to feeding him, which United's isn't and won't be. Will he score goals next season ? Of course he will. Will he beat those 27 PL goals for Everton, I sincerely doubt it. And although Ibra missed some sitters (all CFs do) he also scored goals that Lukaku can only dream of scoring because he doesn't have that repertoire of skills.
As for United challenging... WTF, they finished 24 points behind ! They could maybe have won another 3-4 matches (same can be said for every team taking missed opportunities, nobody takes them all, even sitters) so the only team they'd have challenged would have been us for 4th.
Everton were no more geared to Lukaku than United were to Ibra, (their next highest goalscorer after Ibra in the PL was Rashford with 5 goals).
Can Lukaku beat 25 PL goals this year? I think he can, especially if he takes up the penalty/freekick duties (he didn't have at Everton with Baines there), that Ibra had at United.
As for them challenging, I obviously hope I'm wrong and they fail spectacularly and we can all come back at laugh at this post as Liverpool shock the world. But there 2 reasons why I think United are the main threat:
1) Their opening fixtures - It's a great opening set of fixtures in my opinion:
[xtable]
{tbody}
{tr}
{td}13 Aug 16:00{/td}
{td}1{/td}
{td}West Ham (H){/td}
{/tr}
{tr}
{td}19 Aug 12:30{/td}
{td}2{/td}
{td}Swansea (A){/td}
{/tr}
{tr}
{td}26 Aug 17:30{/td}
{td}3{/td}
{td}Leicester (H){/td}
{/tr}
{tr}
{td}09 Sep 17:30{/td}
{td}4{/td}
{td}Stoke (A){/td}
{/tr}
{tr}
{td}17 Sep 16:00{/td}
{td}5{/td}
{td}Everton (H){/td}
{/tr}
{tr}
{td}23 Sep 15:00{/td}
{td}6{/td}
{td}Southampton (A){/td}
{/tr}
{tr}
{td}30 Sep 15:00{/td}
{td}7{/td}
{td}Crystal Palace (H){/td}
{/tr}
{/tbody}
[/xtable]
That will help them build up momentum ahead of a fixture against us.
2) The Maureen momentum effect - He's shown last year (with the shittest longest unbeaten streak ever) that he's capable of putting a team out there who can't be beaten easily. They went 25+ games. Maureen has shown that whenever he's won a title, it's always been from in front leading early; when his team have been the front runners (not the chasers). Unfortunately, I feel their opening set of fixtures gives him a chance to do that.
3) The main rivals - As I said before, Chelsea are unproven under Conte balancing Europe with the PL, and they've already lost their best 2 players last year (Hazard to injury; Costa on his way out). Man City have signed far too many new players in defence and you need a somewhat settled back four. Tottenham have got weaker. Arsenal are bottlers and we haven't secured our main 2 targets this year. Of course, all this could change in the next 2 weeks.