"He comes from a top side in Dortmund and although he did not play too much last season, he was signed as a young, talented player and to be signed by a club like that shows that he has something good".
The Spaniard left Osasuna to join Dortmund last summer but has featured just seven times.
The 21-year-old striker has agreed a four-year contract to become big-spending Boro's seventh new signing this summer. "He has to show what he has, and hopefully he can enjoy playing here for a long time".
However, Merino's arrival should force a handful of departures from the club.More news: Fresh clashes at al-Aqsa mosque complex
More news: Kim Jong-Nam murder trial to start in October
More news: Scaramucci to Contact the FBI About 'Felony' Financial Disclosure 'Leak'
Borussia Dortmund once again qualified for Champions League football after finishing third in the Bundesliga and the German club see the priority for Mikel Merino as being to get experience and minutes on the pitch elsewhere at the moment.
With Newcastle fans looking for some inspiration in the transfer market, loan deals have been suggested as being on the cards with the club struggling to buy the quality needed.
The Spanish coach has endured a frustrating window and is known to be unhappy with the lack of players moving to St James' Park, after making similar complaints when Newcastle failed to strengthen back in January.