Dallas Cowboys starting quarterback Dak Prescott has labeled this Sunday's game against the rival Philadelphia Eagles as a must-win, but the absence of star left tackle Tyron Smith could lead to their downfall.

For the second consecutive week, Smith is out for Dallas with groin and back issues, according to the NFL Network's Jane Slater. Instead of turning to Chaz Green again, Byron Bell gets the nod at the position.

Green was atrocious at left tackle this past Sunday, allowing Falcons defensive end Adrian Clayborn to set a franchise record for sacks with six. Before that, his career high for sacks in a game was two.

Smith is widely considered to be one of, if not the, best left tackles in the NFL. He's a vital piece of a unit that regularly buys Prescott plenty of time to find receivers down field and opens up holes for their running backs.

Beyond the signal-caller, he's probably the most valuable piece of the Cowboys' offense. Without him, they only managed to score seven points in Atlanta last week. When Smith has played the last two seasons, Dallas has only allowed 1.6 sacks per game. In the four games he hasn't played in, they've given up a total of 1o.

Losing Smith is especially concerning this week against a scary Eagles defensive front. Fletcher Cox, Tim Jernigan, Brandon Graham, Chris Long, and Vinny Curry have given opposing offensive units fits all year.