Nebraska said the tear was revealed during an MRI on Thursday. Day will apply for a medical redshirt to have four years of eligibility remaining.
''It is difficult because B.J. has worked hard in the offseason to get ready for the start of practice,'' coach Tim Miles said. ''We look forward to him beginning his rehab and our goal is to have him ready for the start of next season.''
Day, the son of former Huskers player Bernard Day, averaged 14.5 points and 6.5 rebounds last season for Lincoln Southeast High School.