Q: What is the rule on who carries the Blessed Sacrament in an outdoor procession? Can a deacon carry the monstrance and a priest walk in procession or should the priest always carry the monstrance and the deacon assist?
A: Paragraphs 105 and 106 of Holy Communion and Worship of the Eucharist Outside Mass have the priest carrying the Blessed Sacrament. I assume that’s because he is the presider.
If some practical necessity makes that difficult (the length of the path, the weight of the monstrance, the inabilities of the priest), I see no problem with a deacon assisting, as he is an authorized minister of benediction. But the rubrics don’t speak to it.