BABRAK KARMAL, Moscow's man in Kabul for seven years, is out, reportedly a sick man, and the evident victim of his failure to bring Afghanistan's divided communists together and to make the party a fit instrument for Soviet imperial purposes. Moscow's new man in Kabul is a medical doctor turned politician named Najibullah, 39, who earlier made the Afghan secret police into a weapon of savagery against the resistance. His elevation puts Mikhail Gorbachev's own stamp on a Soviet invasion that has killed, wounded and uprooted half or more of Afghanistan's population and that shows no signs of relenting.

Soviet policy does show, however, some signs of diplomatic movement. Whether these signs are real may become evident as Afghanistan and Pakistan resume ''proximity talks'' under the United Nations secretary general's auspices in Geneva this week. The talks have already covered a certain ground, but the principal question remains what it has always been: whether, even if its demand for an end to ''foreign armed interference'' were met to its satisfaction, the Kremlin actually would pull out the 120,000 troops with which it sustains its client regime in Kabul. Moscow could still provide arms and advisers, but without its own fighting forces in Afghanistan, it would risk losing its whole investment there.

In and around the Reagan administration, there are two views of the likelihood of Moscow's accepting the deal that the United Nations, with gingerly American endorsement, has been working on for several years. The dominant official view, and the one that must be regarded as credible until proven otherwise, is that the Soviet Union is only playing at the talks, since it presumably knows that an Afghan regime even halfway friendly to it could not survive a Soviet troop withdrawal. This view underlies the trend to send more lethal weapons to the resistance fighters, the better to oppose the advanced Soviet arms being used against them. In some Washington quarters, nonetheless, there is a feeling that Moscow might take the U.N. deal and -- an anxiety that Moscow might manage to work its will in Afghanistan through surrogates it left behind.

This seems to us very dubious. A great weight of evidence indicates that the Afghan people would not support those among them who collaborated with the Soviets. The more likely thing the Soviets may have in mind as they play their diplomatic cards is the political scene in Pakistan. There the military has begun to let civilians back into the political arena, and the question of supporting the Afghan guerrillas may yet come under review. Meanwhile, however, the United States has no good-faith alternative to supporting the U.N. talks. If Moscow, to cut its losses at a moment when other demands are pressing heavily upon it, did decide to pull out its troops, the way could be opened to lifting a tremendous burden from a long-suffering people. Diplomacy's work is to keep open this possibility against the moment when Moscow may be ready to test it.