Realistically? Buy no one, maybe a few U-20 or lower prospects. Sell a fullback. I'm not really a fan of mucking things up in the winter window.
If I had a free hand, I would clear Young/Darmian/Miki/Mata/Carrick/Blind. But I'd do it in the summer. I am well aware that one of those players in particular is playing in top form right now. He is nevertheless neither a short nor long-term answer, and I'd get rid of him.
It is obvious that Man Utd are lacking players in departments, and there are also many dead woods in the team. Therefore, I would like to know your opinion on who Man Utd should buy/sell during the winter transfer period.