And Karlsen insists there's bargains to be found on the continent.
He told VG: "Especially now that an incredible number of young, good defenders have emerged - who have speed and are precise with the ball - I think it is strange that Manchester United have not done anything with this position. Both in Germany and France you can find these types,.
"And if you should have taken zero risk and brought in a fully developed player who would strengthen the central defence immediately, both Kalidou Koulibaly in Napoli and Milan Skriniar from Inter are available if you pay enough.
"In my eyes, both (Harry) Maguire and (Victor) Lindelöf are too static in small spaces to be top-class stoppers. They are good when Manchester United are low and the task is to clear and clear the ball away, but when it goes quickly in their own penalty area, I think they get too confused and often fall behind. In addition, they struggle too much in one-on-one situations."