Might as well have written it in the other thread about Pogba too... We will never win the league with a first team full of players who, we hope, they'll eventually become what we need them to be in our system. It doesn't work that way.
You need ready-made solutions and experience in crucial areas all around the pitch. For the ball-playing CB, the tempo-setter, the forward and the 2/3 players behind him we rely on players who may or will reach the desired level but they're not quite there yet. I lost count of how many times yesterday De Gea, Matic and Sanchez threw their hands up in the air in desperation from what was happening around them. If our plan revolves around De Gea being in God-mode every week, Matic running like crazy to cover holes and Sanchez getting on the ball and trying to figure out all by himself, we'll lose these three and we'll hamper the youngsters' careers too.
Not that Mourinho is doing them many favours TBH. I still support him wholeheartedly but sometimes, like yesterday, he saunters dangerously in LvG's territory of stubbornness. Spurs were running rings around us in the middle of the park and yet there we were trying throughout the whole game to play through their press and attack down the middle.
Lukaku is a bulky forward who also has the ability to chase first balls played into space (out wide mostly) but his hold up play isn't top-notch. He's not good at bringing others into play but he can link well with another attacker on the edge of the box and create a chance for himself. You see? Two things he's good at and two others he's not. Now, guess which two things we're telling him to do on the pitch.
If Mourinho wanted a lone forward with the whole package, he shouldn't have signed Lukaku. If he wanted someone less gifted technically but with the experience of having played at the highest level in order to get the job more neatly done and not go missing often, he should have looked elsewhere too. Now, the best thing he can do is devise a plan in which he'll get the maximum of the man he expects to score the goals. At least until Lukaku improves other aspects of his game.