This is a great question that I wished went a bit further. In this scenario I voted for Amir to start, but I think changes are needed to stabilize our 2nd unit... which would involve moving a few of our current starters to the 2nd unit.
Starting Lineup: Horford, Amir, Brown, Bradley, IT (Good ball movement and decision making, with big shot potential and slashing with Brown and IT)
2nd Unit: KO, JJ, Crowder, Green, Smart (Defensive grit, floor-spacing bigs that can clear out creating room for Green to slash... heavy Smart post-ups)