I had this happen, too. Brought up a DL replacement, player got healthy, sent the player back down. Another guy got hurt, and I wanted to bring the same player back up but I had to wait 2 HBD days.
My only question with DL'ing the injured player first, then bringing the AAA player back up, in separate transactions, is this... when you eventually send the minor leaguer back down, will he now be ineligible to have the automatic demotion penalty waived? Because as I understand it, and have seen it in action, the automatic demotion penalty is only waived if the DL on/off and promotion/demotions are processed simultaneously.