I do agree with the comments saying he gets bullied. He doesn't seem to have got to grips with the physicality and aggression of the PL yet. If you watch the Bundesliga for more than 3 minutes it becomes fairly obvious why. Central defenders don't try to physically force you off the ball there, they just mark you.
I think there's two ways to look at that though. Either you just criticise or write him off over it, which is quite silly. Or you see it as something he can work on and improve and accept that you need to allow time for that to happen.
I'd also be more worried if he just wasn't contributing, going into challenges or attempting to do the right things, but he is generally contributing in the way some of our other recent forwards either weren't or had given up doing.
To emphasise the point, I remember Ruud saying that it took him some time to adapt to the league and physicality of it...in his first season, he only managed 5 league goals before mid December (this was masked by him banging in goals in the CL), and he had a pre season, was in the starting line up from the off, and in a team that scored goals for fun. Sesko has had no pre season at all, didn't even get a start until the 4th league game, and then came into a team that had managed 1 goal from open play in the opening 3 league games.
Some strikers can adapt quickly because they are just built right for the demands of the league. A majority though take time to get to grips with it. It's nothing new