First of all, every team that has possession will have problems with counters. We are beating top 6 teams on counters, they have problem as well.
Our problem with counters is based on a few things. First of all, both Maguire and Lindelof are slow. Also, Lindelof is not strong and will often lose duels. Therefore, to stop counters and forwards running behind them, they are making space between them and attacker. As we don't have proper DM, attacker has enough space to control pass to him and include others to counter attack. One of the serious problem is that both our full backs are not fast enough (sometimes look to lazy) to come back to their position in transition from attacking to defensive formation. So, there is wide open space for rival to attack on our two CB who are slow and the only possible outcome is for rivals to have a shot on goal if they execute passing right.
For me, the most important signing in January should be DM. Someone who has positional awareness, and can control space between CB's and midfielders. Even more than n10, as for attacking midfielder we have some options, especially with Pogba fit. Now with McT out, I am not sure how will this work. Matic from the first half season with us would be good, but he is gone. Fred is good recently, but he is not that type of player, and he is not strong enough in duels, and don't have that kind of positional awareness.
Another solution would be adding someone else instead of Lindelof, someone who is fast and strong, and can play closer to attacker as he would be able to recover. Axel is showing promises, but he is often injured.