Isak deal took Dortmund's Tuchel by surprise

Isak moved on Monday from AIK after just 24 games in the Swedish championship for a reported 10 million euros ($10.7m) as Dortmund beat Real Madrid to sign the 17-year-old.

Having watched Isak train on Wednesday for the first time with the Dortmund squad, Tuchel was full of praise for the teenager, who has a contract until June 2019.

"It's a perfect transfer which gives the club incredible long-term planning security, which makes 100 percent sense for Borussia Dortmund," said Tuchel.

Isak has been signed as a back-up for Pierre-Emerick Aubameyang, the Bundesliga's top scorer, who has hinted he may leave at the end of the season.

But according to magazine Sport Bild, the first Tuchel knew of Isak's signing was on Saturday, shortly before Swedish daily Aftonbladet reported the deal was about to happen.

"Everything was decided very quickly. The information flow was very rapid," Tuchel told reporters after training on Wednesday.

"It is very normal with potential transfers of this nature that the coach only has a role late on."

Tuchel said he had no idea about the prodigy when he first heard the name.

"I did not know the player, but it's not possible that I should know of all 16 and 17-year-olds, because it was a potential transfer, in that regard the scouting department and sports director Michael Zorc do a lot of work, of course."

Tuchel says he was also only involved 'late on' in the signings of forwards Ousmane Dembele and Emre Mor before the start of the season.

Dortmund has also bolstered their credentials for unearthing young stars by extending the contract of American 18-year-old Christian Pulisic, considered another football wonderkid, until 2020.

Earlier this month, Isak became Sweden's youngest international goalscorer at 17 years and 113 days and had been linked to Real, but joined the Bundesliga club after an apparent change of heart.

The striker scored 10 goals in his debut season at AIK in 2016.

His talent was acknowledged last September when AIK team-mate Chinedu Obasi said: "He can become the new Zlatan Ibrahimovic" after Isak scored twice in a 3-0 derby win against Djurgardens.