He served as an assistant coach for 25 seasons with the New York Jets, Atlanta Falcons, Denver Broncos, Oakland Raiders, Houston Texans, Seattle Seahawks and San Francisco 49ers.

He went to training camps with the Kansas City Chiefs in 1986, Los Angeles Raiders from 1987 to 1990, and the San Francisco 49ers in 1992 to 1994.

To strengthen the offensive line, the Raiders' general manager, Reggie McKenzie, picked the Texans' right guard, Mike Brisiel, as a free agent.

After a season marked by a switch to a zone-blocking scheme at Knapp's urging, and the resulting ineffectiveness of the Raiders' key offensive threat, running back Darren McFadden, Knapp was relieved of his duties as the Raiders offensive coordinator on December 31, 2012.

[11] On July 17, 2021, Knapp was struck by a motorist while riding his bicycle in San Ramon, California, near his home.