We propose and evaluate the heterostructure based on the graphene layer (GL) with the lateral electron injection from the side contacts and the hole vertical injection via the black phosphorus layer (BL) (P+-PL-PL-GL heterostructure). Due to a relatively small energy of the holes injected from the PL into the GL (about 100 meV, smaller than the energy of optical phonons in the GL which is about 200 meV), the hole injection can effectively cool down the two-dimensional electron-hole plasma in the GL. This simplifies the realization of the interband population inversion and the achievement of the negative dynamic conductivity in the terahertz (THz) frequency range enabling the amplification of the surface plasmon modes. The latter can lead to the plasmon lasing. The conversion of the plasmons into the output radiation can be used for new types of the THz sources.