Dreyfuss has been experiencing "severe muscle spasm" in his back this week, a production spokesman confirmed. The Academy Award-winning actor missed performances April 27-29, but is expected to return April 30.
Performances of the Larry Gelbart comedy, directed by Arthur Penn, play the Barrymore Theatre.
Stepping in for Dreyfuss is understudy René Auberjonois, the veteran character actor who usually plays aged appraiser Crouch in the Volpone-inspired revival.
Charles Antalosky, seen as a servant in the play on other nights, plays the decrepit Crouch while Auberjonois plays Sly.
Sly Fox, opened at Broadway's Barrymore Theatre on April 1.
The play, directed by Fortune's Fool's Arthur Penn, mirrors that of Ben Jonson's Volpone, a British classic which is seldom produced in the U.S.
In it, a miserly rich man pretends to take to his deathbed with the idea of gleefully tricking out of their fortunes every vulture and opportunist who is trying to become his heir.
On April 27, Rachel York was out of the show for the evening. Linda Halaska (Adult Entertainment) stepped in as Miss Fancy.
Also in the cast are Eric Stoltz, Elizabeth Berkley, Bob Dishy, Bronson Pinchot, Irwin Corey, Peter Scolari, Nick Wyman, Jeremy Hollingworth, Robert LaVelle, Jason Ma, Jeff Talbott and Gordon Joseph Weiss.