The Colts fear that Mack suffered a torn Achilles against the Jaguars on Sunday, Tom Pelissero of NFL Network reports.
If that's the case, Mack -- who has an MRI on tap -- would obviously miss extended time, while Jonathan Taylor would presumably be next up to handle the Colts' early down work, with Nyheim Hines also in the mix for added carries on top of his pass-catching duties.