"been saluted imperator by the legions"
In the late 3rd
century BC, the great Roman general Scipio Africanus was hailed as king by the native Spanish troops serving under him. He created the title imperator
for them to use instead, and since then the title had been used by Roman armies to hail a successful leader. Being named imperator by one’s troops was just one of the requirements for a triumph, others including the holding of magisterial imperium
(e.g. praetorship or consulship), withdrawing of troops once the conflict was resolved, and a significant enough victory over a powerful foe. As slaves were not considered a worthy enemy, Crassus should technically not be entitled to a triumph.