It's one of those "legal" vs "able" issues...
As already pointed out, the CDU alone in Alternate Nav is NOT certified for RNP5, so you could not LEGALLY fly an RNAV arrival in normal circumstances.
However, provided the arrival had been entered into the CDU prior to the FMS failure, that route would still be available in the CDU. So, in an EMERGENCY (lost comm, IMC?), you COULD fly that arrival route with nav guidance. You would have to have some confidence in the IRU drift, which you MIGHT get by crosschecking a VOR/DME or ILS/DME fix if available.