${\mathrm{CeAu}}_2{\mathrm{Si}}_2$ is a newly discovered pressure-induced heavy fermion superconductor, which shows very unusual interplay between superconductivity and magnetism under pressure. Here we compare the results of high-pressure measurements on single-crystalline ${\mathrm{CeAu}}_2{\mathrm{Si}}_2$ samples with different levels of disorder. It is found that while the magnetic properties are essentially sample independent, superconductivity is rapidly suppressed when the residual resistivity of the sample increases. We show that the depression of bulk $T_{\mathrm{c}}$ can be well understood in terms of pair breaking by nonmagnetic disorder, which strongly suggests an unconventional pairing state in pressurized ${\mathrm{CeAu}}_2{\mathrm{Si}}_2$. Furthermore, increasing the level of disorder leads to the emergence of another phase transition at $T^{*}$ within the magnetic phase, which might be in competition with superconductivity.

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