In his years with the Rams he played under several different defensive coordinators. Under his first coach the Rams featured the Chicago Bears-style "46" defense that was brought in by recent Tennessee Titans Head coach Jeff Fisher. The next season it changed to a less pressure defense to more of a standard NFL defenses. In 1995 that changed with a new defensive coordinator who introduced the Rams "Jet" defense, where the lineman were in pass rush mode at all times. A couple of years later he began to play in Bud Carson's multiple defenses where Roman was asked to rush the passer more. In the "Jet" defense he was often the only LBer on the field and was asked to cover running backs in coverage and still stop running plays. It was under Carson's defense that Phifer would play the equivalent of a safety on one play and the equivalent of defensive end on another play and a run-stuffing linebacker on another and a pass-cover linebacker as yet a fourth way to play defense. In 1998, he had a career-high 6.5 sacks and 71 tackles with an interception a forced fumble and a fumble recovery on an improving Rams defense, with his career high in sacks being the result of playing standup defensive end some of the snaps.