FOXBOROUGH, Mass. -- What can the New England Patriots expect when it comes to the availability of second-year wide receiver Malcolm Mitchell in 2017?

That question came to mind on Wednesday as Mitchell was absent from practice for the second day in a row, which continues an on-again, off-again pattern that reflects how he's still managing knee issues.

With 53-man rosters to be formed by Sept. 2, and the season opener Sept. 7, Mitchell's availability currently looks like a moving target. That uncertainty represents a challenge of sorts for Bill Belichick, director of player personnel Nick Caserio and Co., as they ultimately form the team's initial roster.

Consider that Mitchell, whose impressive work in the second half of the Super Bowl was a vital part of the team's comeback, was held out of all spring practices. Then when training camp began July 27, he was a limited participant who spent more time on the rehabilitation field.

Mitchell was then held out of the preseason opener, before participating in team drills at practice for the first time on Aug. 14. His workload remained at that level through joint practices with the Houston Texans last week, and then he played 15 snaps in Saturday's preseason loss at Houston, finishing with two catches for 13 yards.

At times, Mitchell reached down for his right knee during the game.

Now he's out of practice again.

When looking at the Patriots' wide receiver depth chart, the 6-foot-1, 200-pound Mitchell is No. 4 or 5, behind Julian Edelman, Brandin Cooks and Chris Hogan, and right there alongside Danny Amendola. That's a well-stocked group, which doesn't include special teams captain Matthew Slater, who occasionally takes reps at the position.

Devin Lucien (2016 seventh-round pick) and 2017 undrafted free agent Austin Carr headline the receivers behind them vying for roster spots.

Mitchell's roster spot is considered safe, but less certain at this point is how much he'll be available to the club in the early going. In a week that the Patriots are adopting a bit of a regular-season type approach, it is notable that Mitchell hasn't been on the field with his teammates, and thus also won't be with them for Friday's preseason game in Detroit.