B757-200
Flight Crew Air Transport Pilot (ATP)
Time Pressure
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL
Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed in Emergency Condition
Aircraft
At cruise received cabin altitude warning.  Cabin was at 10;000 ft.  We descended from FL280 to FL100 then further descent to 5;000 ft. [Advised ATC] and landed. QRH complied with O2 masks. Right pack [failed] to maintain pressurization; left pack had been deferred.
B757 Captain reported receiving a cabin altitude warning during cruise.