Huntelaar is a big surprise in the table, in my opinion. He is a good striker but not good enough to be in the top of greatest strikers.
He maybe isn't a world class striker but he is very deadly in the 16.
He could easily scored way more goals for the national team but he had the bad luck that he had to challeng Van Persie for that spot. The coach always choose for Van Persie while Huntelaar scored more easily. I think he just had some bad luck he wasn't a 'big name' back then.
For Schalke he still could be a big help. Maybe he can't play all matches full but he still got the drive.
Huntelaar was one of the best strikers in the whole world at his peak if you ask me, people always undervalued him but he was very capable of scoring and went as high as playing for Real Madrid at a very high level for few seasons. However even I have to realize that he is 37 years old now, sure Ibrahimovic made news when he is 40 years old and still scored bunch of goals, but even he slowed down and nowhere to be found now, so Hunterlaar is not going to be that much of a change for any team.
To be honest all he has to do is try to not relegate with the team, that is not really a huge problem, and it is not a big standard to keep neither, do not lose every single game and that's it. Obviously it will be a bit of challenge, just nothing huge, if he comes in and scores like 7 goals rest of the year, that would be a marvelous investment for Schalke as well.