Pietersen is a right arm off spinner i.e. an orthodox “finger spinner”. He uses his fingers to spin the ball rather than his wrist. An injury to the little finger on his right hand shouldn’t be a problem when batting and guarded in a glove but wrapping it around the ball over and over is probably not the best idea until it is healed.
That and the constant rush of blood and swelling (in the hand!) associated with bowling long spells is probably not good for an injury either.
He is in the squad for his batting anyway.
Either that or all this pretending to be a pom has turned him into a big girl’s blouse.