Peter Kisfaludy previously coached Bergvall in Brommapojkarna's youth team - and he was active in Djurgården in connection with the midfielder's domestic transfer.
Kisfaludy says he could see early on that Bergvall was something extraordinary.
"I worked with him from the time he was nine until he was 14. Right after the first workout with him, I thought 'this is something special'," he told The Sun.
Kisfaludy has managed to train several great talents over the years in Brommapojkarna - but Bergvall stands out from the crowd.
"We have had (Viktor) Gyökeres, (Dejan) Kulusevski, John Guidetti and Ludwig Augustinsson, but Lucas has the most talent of all," he says.
"He's always had a great technique. And he reads the game, he always has his eyes up and keeps track of what's going on. He always knows what the next step is."