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New York Jets quarterback Sam Darnold is "not a lock" to return in Week 5 against the Philadelphia Eagles.

The second-year quarterback, currently sidelined with mononucleosis, will undergo more testing on Monday, according to ESPN's Chris Mortensen:

The 0-3 Jets have a bye in Week 4, and the team was hoping Darnold wouldn't be forced to miss any additional time with the illness, especially considering that a potential 0-4 start would be a major blow to the team's already fading postseason hopes.

Darnold, 22, missed the team's 23-3 loss against the Cleveland Browns in Week 2 and its 30-14 loss to the New England Patriots in Week 3. His backup, Luke Falk, offered a mixed bag in relief.

In Week 2 he was solid if unspectacular, completing 20-of-25 passes for 198 yards, though he couldn't lead the team to a touchdown. But in Week 3 he was exposed by the Patriots, completing just 12-of-22 passes for just 98 yards and an interception.

Obviously, the Jets aren't going to be as good with a backup quarterback at the helm. But zero touchdown passes through two weeks is a major concern.

And the Eagles' secondary has proven to be more than vulnerable this season, giving up 323.8 passing yards per game (31st in the NFL) and nine passing touchdowns (tied 29th) through four games. The Jets could have success against Philly's secondary, though Darnold seems far more likely to expose that unit than Falk.

But climbing out of an 0-3 hole won't be a small task and will require more than just an upgrade at the quarterback position, as Darnold told reporters on Monday, per Manish Mehta of the New York Daily News:

"Obviously, we need to fix some details at other spots as well. We're not blind to that. It's just about going out in practice, in the meeting rooms and walk-throughs and being able to fix all that stuff... and not being shy about it either. Not worrying about hurting anyone's feelings. It's about getting this stuff right. And we're going to get it right. It's just a matter of time."

Still, the Jets are more than aware of how important it is to get Darnold back on the field.

"It'd be nice to get [Darnold] back," head coach Adam Gase said Monday. "That always helps things, because he makes some bad things go away."

Whether that happens in Week 5 remains to be seen.