I don't think Putin wants to integrate Novorossia into Russia, as it would just mean moving the East/West border. He'd be more interested in having a friendly, or at least, neutral buffer state with no NATO bases on its soil. Think Belarus.
If Putin sits idle, he will not get the buffer state he wants. The Kiev junta will invade Novorossia, and half the population might end up in Russia as refugees. The Novorossian army lacks aircraft and tanks. They won't be able to hold on for much longer.