For a guy as tall and heavy (I know it's not fat) as him, I'd say he doesn't really do a great job. He actually often gets bullied by the opponents defender, Colley against Sampdoria, Koulibaly against Naples, Chiriches against Sassuolo. He doesn't win a lot of balls in the air. The only time where you can see that he is a bulky player is when he receives the ball with his back to the goal.
But that's not my issue with Lukaku. My issue is when in your team you have people that run a lot, like Brozovic, Barella, Eriksen, Lautaro or Candreva, the amount of casual walking Lukaku does, hurts my eyes. He doesn't press, the only moment where he runs is when he wants to receive the ball. He never fights to win it back, in this case I prefer Lautaro and Sanchez' style of attackers because those are the one that make the team look dangerous when pressing.