By letter filed with the City Clerk the Manager shall designate, subject to approval of the Council, a qualified City administrative officer to exercise the powers and perform the duties of Manager during his temporary absence or disability. During such absence or disability the Council may revoke such designation at any time and appoint another officer of the City to serve until the Manager shall return or his disability shall cease; however, in the event that the Manager has failed to designate an Acting Manager, the Council may by resolution appoint an officer of the City to perform the duties of the Manager until he shall return or his disability shall cease.