Help says this. "Playoff Eligibility Note - In order for a player to be eligible for a spot on the big league playoff roster, he must be on the 40-man roster prior to the date rosters expand. Any player on the 40-man roster but not on the big league roster is then eligible to replace any player injured before or during the playoffs at the beginning of each round."

So, I have an injured SP(my best). He is not on the DL. If I was to replace him on the playoff roster for the first series I would have to promote a guy from AAA. What would I do with the injured SP if I made him ineligible? Would I have to demote him? Wouldn't want to do that because he'd take a further hit to his ratings. 25 man roster is full with him on it. I didn't put him on the DL because he'll be ready for the next series(should we get there) and I would really need him then.

Putting a player on the DL does not remove him from the 40, technically.   He won't show on the 40, if it's the 60 DL, but, when you remove him from the DL, he'll be on the 40.  Hope that makes sense.

Turns out I am able to make the SP ineligible and promote someone from AAA and make them eligible without putting the first guy on the DL.

Yes, you can remove him from the 25 man playoff roster without putting him on the DL.  You would have to promote the AAA guy to the ML roster and then add him to the playoff roster.

If your pitcher is then ready for the next round (assuming you make it), you can then add him to the playoff roster and remove the other guy in between rounds.