Carl Gross, Whitney's former coach at Jackson High School, said MU defensive coordinator Matt Eberflus called him yesterday and broke the news of Whitney's departure.
"I'm completely shocked," Gross said. "I thought everything was great. I'm disappointed for him."
Gross said he hasn't spoken to Whitney to find out his reasons for leaving.
"The last time I talked to him was during the Christmas break, and he was really looking forward to this season," Gross said. "He was here lifting and working out every day, so I don't have a clue why."
In a speaking engagement Monday in St. Louis, Missouri Coach Gary Pinkel spoke highly of Whitney and the contribution he might make next season.
Whitney came to Missouri as a high-profile running back recruit, and he played in the Tigers' first two games last year before suffering a shoulder injury. He sat out the rest of the season as a medical redshirt.
"Any time you go from playing a lot to the scout team, it's going to be difficult," Gross said. "But a lot of people go through that and do well. He just didn't deal with that very well."
Mother Carla Whitney said she talked to Whitney on Monday night, which is when he made the decision.
He cleaned out his dorm room in Smith Hall on Tuesday afternoon and stayed with a friend from high school Tuesday night, she said.
"Ever since he injured his shoulder, he just hasn't been happy," Carla Whitney said. "You can't stay and function when you're not happy."
Whitney, who entered MU as a running back, played in two games in 2002. He ran one time for 4 yards in the season opener against Illinois and four times for 3 yards against Ball State.
He also returned two kicks, one for 17 yards against Illinois and another 28 yards against Ball State.
At Jackson (Mo.) High, Whitney rushed for 5,027 yards and scored 90 touchdowns. He holds the Missouri state record of 9.3 yards per carry in a career and ran for 463 yards in one game, another state record.
Before the season, Whitney lamented being on the scout team after being a star in high school.
"A whole lot less reps on that fourth team," he said in August. "It's a whole lot different going from first team in high school to fourth team. I definitely don't want to be down there for very long."
A mild concussion he suffered during summer practices Aug. 16 also hampered Whitney. It kept him out of three days of practices and was his third concussion.
"It probably doesn't feel as bad getting the concussion as knowing that you're not going to be able to play," he said at the time.
On Feb. 5, Pinkel said Whitney had moved to wide receiver.