Texas Tech defensive coordinator Matt Wallerstedt abruptly resigned Thursday after sources told ESPN.com that the coach was suspected of being under the influence of an unknown substance while on campus.
The source, who told ESPN.com earlier on Thursday that Wallerstedt had been fired, said the coach was sent home from the football facility on Monday.
"I have submitted my resignation to Coach Kingsbury effective immediately because I want the best for the Red Raider program, and this will allow them to go in a different direction on defense," Wallerstedt said in a statement. "I love our players and this school and have only their best interest in my heart and mind. I wish Coach Kingsbury every success this season."
The Red Raiders surrendered 438 rushing yards in a 49-28 loss to Arkansas last weekend, but the team's on-the-field performance was not related to Wallerstedt's dismissal, sources said.
Wallerstedt joined the Texas Tech staff last season, coming over from Texas A&M along with head coach Kliff Kingsbury.
"We appreciate all Coach Wallerstedt has done at Texas Tech and we wish him the best," Kingsbury said in a statement.
Wallerstedt will be replaced by co-defensive coordinator and linebackers coach Mike Smith, who played for the Red Raiders and graduated from Texas Tech in 2004. Matt Brock has been elevated from defensive quality control coach and will now coach the linebackers.
Smith will be Texas Tech's seventh defensive coordinator since 2007.
The Red Raiders are off this weekend before traveling to play Oklahoma State on Sept. 25.