Corpsejack menace, master biomancer

[Wackaman9001]

If I have a {master biomancer} in play and cast a {corpsejack menace} , is he going to be a 6/6 or 8/8?

[Keyeto]

He will be a 6/6, since his effect isn't in place before he enters the battlefield.

614.4. Replacement effects must exist before the appropriate event occurs—they can't "go back in time" and change something that's already happened. Spells or abilities that generate these effects are often cast or activated in response to whatever would produce the event and thus resolve before that event would occur.

And to bust out another ruling:


614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c-d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply, check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see rule 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent's characteristics on the stack (see rule 400.7a), and continuous effects from the permanent's own static abilities, but ignoring continuous effects from any other source that would affect it.

To put it simply:
The replacement effect will only be applied to the permanent itself, when it only affects itself. If the replacement effect applies to a subset of permanents (including itself), it won't affect itself.

RESOLVED

[Wackaman9001]

If I have a {master biomancer} in play and cast a {corpsejack menace} , is he going to be a 6/6 or 8/8?

{Master Biomancer} would add 2 +1/+1 counters. {Corpsejack Menace} would double it to 4 +1/+1 counters. It would end up as an 8/8 and have the Mutant Subtype.

That's what I thought, and it's how I played it at FNM, but keyeto's rulings make sense