They need to buy players, but they also need to sell players. The squad needs a complete overhaul. And if that happens, it's difficult to see them all gelling as a team immediately. It will probably take quite some time, and Man U might have to get used to being outside the Champions League places next season, too. The big question at the moment I suppose is how much of a draw are they to prospective incoming signings? Would the world's best players want to join a team that's not playing CL?
Yes, there must be an overhaul in the Man United squad, so the burden that must be carried by Ten Hag is quite large because no one knows how long he will be given time to fix Man United. Therefore, Ten Hag must take action as quickly as possible and the top priority should be fixing Man United's defense and maybe looking for a defensive midfielder who could play the water carrier role.
With the current conditions, it might be difficult for Man United to get world-class players. It's not just because they don't play in Champion League but it could be due to limited funds, so there really have to be some players to sell.