Kobel will be expected to become Dortmund’s No. 1 ahead of Marwin Hitz, who last season beat out Roman Burki for the starting job.
Barcelona have signed Argentina forward Sergio Aguero on a deal until 2023, the Catalan giants announced on Monday.
The 23-year-old Englishman signed a four-year deal with the German champions, the team said Thursday.
The England captain reportedly wants his future resolved before the delayed Euro 2020 starts on June 11.
Mateta has scored 10 goals in 17 appearances for Mainz in the Bundesliga and German Cup this season.
Juventus has ruled Serie A for nine consecutive seasons and Milan has finished no better than fifth over the past seven years.
Fulham has invested heavily in its playing squad, only to start without a point from two games before its first league win on Sunday.