Any process that stops, prevents or reduces the frequency, rate or extent of K48-linked ubiquitination, a protein ubiquitination process in which a polymer of ubiquitin, formed by linkages between lysine residues at position 48 of the ubiquitin monomers, is added to a protein. K48-linked ubiquitination targets the substrate protein for degradation.
negative regulation of protein K48-linked ubiquitination