SDS, please do not allow starters to come in as relievers. This makes for bad gameplay, less strategy, and unrealistic pitcher usage in DD.
A lot of your player base is already tired of seeing Kluber either starting or coming out of relief every game. I actually do not mind this yet, because as a player I can work counts, get Kluber tired, and also get used to the break on all his pitches.
However, in the future we will have a huge problem on our hands. Once our rotations are full of 99s and players we can trust, this game will be unplayable in ranked seasons. I already know how it will go, because I will use the same strategy.
Once I have 99 Kershaw, 99 Kluber, 99 Gibson, 99 Seaver, and 99 Ryan, I will be able to use all 5 pitchers every game if I wanted to. I will throw 2 innings with Kershaw, 2 innings with Kluber, 2 innings with Gibson, then go to my pen with 99s. Next game, just repeat the process with Seaver & Ryan and I'll never have a pitcher that gets below 90% stamina EVER in any game.
How will this affect gameplay?
Working counts will be pointless, as you will never get to face a halfway tired pitcher.
The PCI will always be at its smallest. Once again, since a pitcher won't ever be tired you will never get the opportunity to allow the PCI to grow
You will never be able to get used to a delivery or used to the break on each pitch. Before the 2nd time around the lineup there will already be a new pitcher in the game.
There will be no strategy or risk/reward for letting pitcher go deep in a game. Just pinch hit every PA with the pitcher.
I promise, this will be what the game becomes if we continue to let starters come out of the pen. Please, see how this will affect the game negatively and make proactive changes.