DON'T PANIC. As we discussed before, this is a different beast than it was a couple of years back when Semaj declared for the draft. The deadline to withdraw from consideration for the draft has been pushed back, so Trevon's declaring is different than Trevon's leaving.
This basically means that Tre will get to go through the combine and be evaluated by NBA scouts. He will also get to work out for a team and receive feedback from them. After he has gathered that info, he will have the opportunity to make a decision regarding his next steps.
I would anticipate that this will lead to Trevon's return to Xavier. He's not an above the rim athlete, and I don't think his game is so refined right now that the NBA just has to have him. Until such time as it comes out that Trevon has hired an agent, consider this a fact-finding mission and nothing else.