I like Monroe, but with Horford on board, I think the way to go now is run with Amir / Horford in the frontcourt and protect cap space for next summer, when the Celts can go after Blake Griffin, Cousins, and/or Westbrook.
This next season is about showing progress, making Thomas / Crowder / Horford look great, making the young bench guys look like they have lots of potential, and keep the path clear for a top 10-15 player to join the Celts and make this team a contender for a few years.