Mickelson missed the projected cut by four shots, shooting 77-71.
''I don't feel bad about the physical game, the ball coming off the blade,'' he told reporters. ''The swing is on plane. I need better mental focusing, seeing the shot before I step into it.''
The MC was only Mickelson's third of the year, but second in a row, following his early exit from the Masters.
DeChambeau, who finished T-21 in the Masters, then followed with a T-4 finish in his much-anticipated debut as a professional at the RBC Heritage, shot 74-73 in San Antonio. A California native, he went to college at Southern Methodist University in Dallas.