Are managers even more knowledgeable about footballers not within their immediate proximity than your average football fan is? These guys spend most of their time on the training ground, dealing with their own players. What makes anybody think that they have the necessary energy and desire to keep up with dozens of players from other teams and leagues in their limited spare time?
"Back the manager by signing players that are handpicked by him" has to be one of the most misguided shouts in terms of squad building. The ultimate goal should be the exact opposite: To establish a recruitment system where manager has minimal impact, because there are already specialists who are employed specifically for this purpose.