On Aug. 29, 2017, Jürgen Klopp’s phone pinged incessantly. Liverpool had reached a pre-agreement with RB Leipzig for the transfer of Naby Keita, effective the following summer, and congratulatory messages were piling in.



The notifications varied from Germany legends to the manager’s former players and high-ranking Bundesliga officials, with their texts all containing the same core communication: the Reds had pulled off some coup.



At the time, Keita was—as per Klopp’s own description—the “player of the league” and “the flier,” so the flood of WhatsApps and texts extolling the midfielder’s defensive and offensive excellence were not surprising. There was nothing Liverpool didn’t know about the dynamo, who had the ability to dribble past an opponent or dispossess them with equal adroitness.



The scouting of Keita was so extensive and the conviction about him so crystalline that the...