What’s wrong with Jeremy Maclin?

The young Philadelphia Eagles receiver has yet to participate in the training camp due to an undisclosed illness, which has a lot of people wondering what’s up.

Is the team hiding something? Is Maclin hiding something? Or do neither know what’s wrong with him? Maclin reportedly contracted mononucleosis earlier this year, but it hasn’t been made public if this ailment is related to that one, or whether that’s even known as this point.

When fans see that important players are sitting out due to “illness,” they usually breathe sighs of relief. We seem to have a natural impression that illnesses are less severe or harmful than injuries. Maybe it’s because we equate “illness” with the common cold and the flu. Suck it up, man!

But something beyond the the sniffles is bothering the 19th pick from the 2009 draft. Maclin has returned to his hometown of St. Louis to undergo tests to identify his illness, according to local Philly radio host Howard Eskin, who notes that there’s a real concern he won’t be ready to start the season.

While it goes without saying that Maclin’s health comes first and it’s safe to assume that football has become secondary in his world, this is a major blow to a kid who caught 70 passes and piled up 964 yards to go with 10 touchdowns in his sophomore season with the Eagles.

And while his game isn’t as multifaceted as DeSean Jackson’s, Maclin might have been the team’s most complete receiver in 2010. Calling him a great complement to the two-time Pro Bowler Jackson, who caught four fewer touchdown passes than Maclin last year, doesn’t do Maclin’s contributions justice.

The Eagles bolstered the receiving corps with the addition of Steve Smith yesterday, but Smith is expected to miss a large chunk of the season due to knee issues. That leaves Jason Avant to fill Maclin’s shoes between now and whenever Maclin is feeling well enough to return.

And while Avant is a stellar No. 3 or No. 4 option, he can’t be relied upon to do what a healthy Jeremy Maclin could.