The description I've heard of repairing a torn Achilles is, "sewing two mop heads together." Expecting 1K+ yards after than injury like that is very optimistic.
It's possible he comes back where he was last year, but that is not the most likely outcome. I would guess 500-700 yards, and would be very happy to be proven wrong.
Quick excerpt from "Epidemiology and Outcomes of Achilles Tendon Ruptures in the National Football League"
"Results: 78 Achilles tendon ruptures were identified in professional football players during the 2010-2015 NFL seasons. 58% of these injuries occurred during the preseason. Of those that suffered an Achilles tendon rupture, 26% did not ever return to play in the NFL. Players who did return to play in the NFL took an average of 9 months to recover after the date of injury. Across all
positions, there was a net decrease in power ratings by 22% and a net decrease in approximate value by 23% over 3 years following player return after Achilles tendon rupture. Across all positions, running backs saw the biggest decrease in production with a 78% decrease over 3 years post-injury in both power ratings and approximate value."
My best guess is that Potts is the clear #1 by the end of the season. This is an over-use injury that's caused by improper training and load management. Mo will be lucky to recover from it, and I hope that Fleck learned his lesson about load management and running back rotations. There's a reason that the best offenses in the NFL don't "feature" RBs. No player other than a C or QB should be touching the ball 30+ times per game, especially that early in the season.