Leicester City manager Claudio Ranieri said Demarai Gray's performance against the club in pre-season prompted the Italian to sign the winger.
Gray, 19, caught the attention of Ranieri when Leicester faced off against Birmingham City in a friendly back in August.
Ranieri said Gray was already on his radar after watching clips before the pre-season clash, but was really impressed by how the teenager performed against the Premier League outfit.
"I watched some matches before playing against Birmingham and already I saw he was very good. But when we played against him I thought: 'he is fantastic'," Ranieri said.
"He was a very good player. He was fast, dribbled well and passed the ball well, he was a hard worker.
"We tried to buy him at the beginning of the season and it wasn't possible but now it is possible so we bought him.
"I saw my team and thought about how we could improve. I am sure Demarai Gray will improve my team.
"I think he is also for now. He can help me to give some rests when some wingers are tired."
Gray, who has represented England at youth level, is expected to make his Leicester debut in their FA Cup third round clash with Tottenham on Sunday.