The day has arrived and as of this post I have NOT read the comic. We've all seen the previews and the subtle clues that our favorite Lunar Legionnaire will be having delusions and believing that he's Spider-Man, Captain America or Wolverine. We've seen in the previews that Marc is talking to these men as the series opens. Later, we see Marc taking on Mr. Hyde, a villain of vast superhurman strength who should honestly swat him around like a fly.
My theory is this: Mr. Hyde's brutal assault causes Marc some form of mental trauma/relapse to his previous mental problems. The last men he spoke with would be prevalant in his fragile mind. Therefore, voila he starts thinking he's actually them.
This may be the ramnbling of a man that also needs mental help (I mean myself) but I think it could actually happen.