Lowry, who scored a career-high 43 points and played 43 minutes in the Raptors' victory Friday over the Cleveland Cavaliers, had played in all of Toronto's 57 games this season prior to Sunday's contest.
"He played so many minutes the other night," Casey told reporters before Toronto's 114-101 loss to Detroit. "We needed all of his 43 that he gave us, so we're going to rest him tonight to make sure going forward we don't wear him down."
The Raptors fell to 39-19 with Sunday's loss but remain two games behind Cleveland (41-17) for the top spot in the Eastern Conference. Toronto is five games ahead of the second-place Boston Celtics (35-25) in the Atlantic Division.
Lowry is averaging a career-high 21.4 points per game and a career-high 37 minutes this season.
Cory Joseph replaced Lowry in the starting lineup Sunday but was plagued by early foul trouble and finished with 10 points and two assists in just 20 minutes.
The Associated Press contributed to this report.