Four weeks into the season, the victory lap that once seemed assured for the New England Patriots appears somewhat in doubt.

The defending champions have a problematic defense, a unit that has primarily cost them two games already in 2017 and nearly cost them a third. The unit has given up a league-worst 31.7 points per game and ranked dead last by Football Outsiders' DVOA. The 33 points allowed to the Carolina Panthers -- owners of the less-than-pedestrian 25th-ranked offense by DVOA entering Week 4 -- further underscores the severity of New England's defensive struggles. Despite the presence of experienced defensive coordinator Matt Patricia and historically great head coach Bill Belichick, the Patriots can't seem to stop anyone in 2017.

There's no question injuries have played a role. Dont'a Hightower, the heartbeat of the unit, suffered an MCL injury in the season opener and missed the next two games. A groin injury sidelined cornerback Eric Rowe for a game and forced him out of this week's tilt with the Panthers. Top free-agent acquisition Stephon Gilmore hasn't missed a start, but a groin injury of his own has limited his impact. For a unit that lacked proven depth at several spots entering the year, these health issues have taken their toll.

But that doesn't account for all of the Patriots' shortcomings. The pass rush resembles a tire fire, registering a woeful 5.5 percent adjusted sack rate (25th in the league) at the close of September. While the coaching staff adjusted well to the departures of Chandler Jones, Jamie Collins and similar impact players in the past, the dearth of pass-rushers has grown into a problem too large to scheme around. The team needs to hunt for solutions down the depth chart and perhaps outside the organization.

Entering the week, rookie defensive end Deatrich Wise led the team with nine quarterback hits. Yet despite such production, the Pats played Wise on just 38.7 percent of the available defensive snaps. With only Trey Flowers delivering consistent pressure among the starters, New England needs to increase Wise's workload significantly and overlook the rookie errors that follow.

One possible solution: defensive end Dwight Freeney, a one-time Patriots killer as a member of the Indianapolis Colts, who has sat on the open market awaiting a chance to jump aboard a title contender. Freeney lost the burst that made him an All-Pro long ago, but as he demonstrated in recent seasons with the Atlanta Falcons and Arizona Cardinals, he remains a viable threat in limited snaps. He could slide in as a tertiary pass-rusher in the mold of Chris Long.

If the cornerback position continues to hinder the defense, Belichick could call up a familiar face. Darrelle Revis, the future Hall of Fame cover man who won a Super Bowl with the Patriots in 2014, has not received much interest from teams since the New York Jets released him just before the start of the new league year. Revis' physical gifts have deteriorated since he left Foxboro, and he can probably only operate in the slot at this point in his career. The offset language in his contract with New York could also complicate any negotiations. However, Belichick can put Revis in positions to tilt the field, something the defense badly needs in its current state.

Ultimately, the burden to overcome an underwhelming defense will fall on the shoulders of Tom Brady. The offense remains potent, averaging 32.3 points per game. New weapons Brandin Cooks and Rex Burkhead have already integrated into the system, adding to a supporting cast that rates as one of the best of Brady's career. The unit has performed at or above expectations by any reasonable metric. However, if the defense continues to struggle, the Patriots offense might have to somehow deliver its most prolific season yet to counterbalance it.

While the Pats face as many defensive problems as any other team in the league, there is still time to fix their weaknesses and alter the trajectory of the season. No team possesses a brain trust better suited to tackle such issues, and Belichick and company have rebounded from dire situations before. But for a defending champion that generated talk of a 16-0 campaign just four weeks ago, the myriad problems suggest this season could still be a slog.