FOXBOROUGH, Mass. -- In a span of two days, the New England Patriots finalized plans to have Dion Lewis undergo a second surgery on his left knee and released eight-year veteran Donald Brown, which highlights why the club figures to explore the waiver wire and trade market at running back.

The position was already viewed as a need entering training camp, a result of the Patriots not drafting a player at the position.

One wild card remains undrafted D.J. Foster of Arizona State, who hasn't played in the preseason because of an undisclosed injury but appeared to increase his practice workload on Monday. Foster was a priority free-agent signing whose $30,000 in guaranteed money was second highest among the team's undrafted players. He fits into the "passing back" mold of Lewis and top replacement, James White.

As it stands now, this is how the Patriots' lighter-than-desired running back depth chart breaks down:

Power back

LeGarrette Blount

Tyler Gaffney

Joey Iosefa

Passing back

White

Brandon Bolden

Foster

The veteran free-agent market includes Fred Jackson, the longtime Buffalo Bill who was courted by the Patriots last year as a free agent. Jackson signed a one-year deal with Seattle instead. A lingering question with the 35-year-old Jackson: How much does he have left?

Other options, such as Roy Helu, have health-related questions. Helu was released by Oakland earlier in August with an injury settlement.

It highlights the importance of Patriots director of pro personnel Dave Ziegler, who must be up to date on each NFL team's depth chart so the Patriots can pounce on potential opportunities, such as last year when then-director of pro personnel Bob Quinn played a role in the team acquiring defensive tackle Akiem Hicks from the New Orleans Saints.

The Tennessee Titans, for example, are deep at running back with the likes of DeMarco Murray and Derrick Henry, so once-highly touted Bishop Sankey could become available.