Realmuto's leadership on the field and in the clubhouse (if he's away from the club) will be missed.
It remains to be seen if Stubbs/Marchan can adequately handle managing the pitching staff and holding down the running game.
At the plate...J.T. has a .720 OPS this season. That's more than we can realistically expect from Stubbs or Marchan, but it's not as Realmuto was leading the club. Of the six "qualified" players (based on plate appearances), J.T.'s OPS ranks fifth, ahead of Castellanos, but behind Harper, Bohm, Schwarber, Stott. And if you linclude "non-qualified" players, there are nine Phillies hitters with better OPS than Realmuto (this includes Trea Turner, Kody Clemens and Brandon Marsh on the IL). So, Realmuto is valuable, but he's not a critical offensive contributor, ranking tenth in OPS, ninth in OBP, ninth in SLG.
If his replacements can handle the defensive roles (receiving, running game), J.T.'s absence should be manageable, particularly when the other guys on the IL (Turner, Marsh, Clemens) return.