Why in the world would you assume that Stukov wouldn't be re-introduced as a brand new concept? Of course he would!
Is that more work than introducing Matt Horner? Sure. But the reveal itself could be fun, as opposed to tedious, as long as the circumstances force the characters AND the audience to feel as though they're on the same page.
Hyperion bridge.
MATT
We're receiving a transmission from the incoming shuttle, Sir.
The image of a gray-bearded man in his 50s appears on the viewscreen.
MAN
This is... Alexei Stukov. You've no idea how difficult it was to get a hold of you, Commander Raynor. May I come aboard?
Tychus looks over at Raynor. Raynor's face is a mask, his thoughts indecipherable.
TYCHUS
Stukov? Vice Admiral of the fleet from Earth Stukov?
Tychus looks over at Matt, who's obviously fidgeting and shooting uncomfortable glances at Raynor.
TYCHUS
(cont'd)
They reported his death... they even hacked the UNN to air footage of his funeral, for God's sake!
Raynor sighs.
RAYNOR
No, Tychus. The UED was wrong. Their man was still alive -- if you can call what he went through living. Y'know, I'd like to say it'd be a lot simpler if I let the Vice-Admiral speak for himself... but the truth is, some stories don't make sense no matter how many times you hear 'em.
TYCHUS
(knowingly)
Or see them first-hand, huh?
RAYNOR
Or see them first-hand. Open up the front door, Matt. Let's find out why the Vice-Admiral's come calling.
Now of course that's FAR from ideal... this is a lazy hack-job that I put together in 5 minutes, with no thought as to what role Stukov actually plays in this story. But already you can see how we're seeing things through Tychus's eyes, and if we find something hard to believe, it's a safe bet he will too, and say so. Of course while this opening might work, the only (obvious) way to resolve it is through scenes of exposition, where Stukov and Jim and Tychus sit in a circle and explain just what happened... this isn't ideal.
Hence Stukov's arrival could instead be at a time when explanations aren't possible because Raynor/Stukov/people are in trouble and action is called for. And instead of expository dialogue, we could eventually get Stukov's story told in a cinematic -- ie, nightmares he experiences in his sleep. Like mr. peasant suggested at the bottom of page 1.