97 Still, though it is clear from what We have said that the Mass is offered in the name of Christ and of the Church and that it is not robbed of its social effects though it be celebrated by a priest without a server, nonetheless, on account of the dignity of such an august mystery, it is our earnest desire - as Mother Church has always commanded - that no priest should say Mass unless a server is at hand to answer the prayers, as canon 813 prescribes.
Source: Mediator Dei (Vatican.va)