Allofs told the local Weser-Kurier newspaper Tuesday that ''everything is arranged between Werder and Arsenal.''
An official announcement is expected on Wednesday, following completion of some final details between the clubs.
Allofs said Bremen will hold a news conference on Wednesday.
German media says Arsenal will pay around ?10 million ($14.5 million) for the central defender who is entering the final year of his contract with Bremen. The 26-year-old Mertesacker will reportedly receive a contract until 2015 worth ?4.7 million per season.
Mertesacker has 75 caps for Germany.