An official announcement is expected Monday.
DeCuire spent the past six seasons on the staff of coach Mike Montgomery, who retired in April and publicly endorsed DeCuire as his replacement. Cal wound up hiring Tennessee's Cuonzo Martin.
Tinkle got the job at Oregon State, for which DeCuire was also in the running.
DeCuire played guard at Montana from 1991 to 1994 and holds the school record for career assists.
Montana loses two of its top scorers from a team that finished 17-13 last season.