That battle between Chinese and Arab forces, which resulted in the Chinese retreat from Central Asia. Toynbee cited this as one of the great lost opportunities, I think it was the "Forfeited Birth-rite of Far Eastern Christianity" which I rather disagree with, that if the Arabs had lost, then Nestorism or whatever would succeed. But if you have China win, then you keep Chinese cultural supremacy over Central Asia, and probably stop the spread of Islam into the Oghuz Turks. If you have a strong enough Chinese cultural presence, then you could end up with Buddahism being the main religion, perhaps the Tibetians get in there, and give the Turks some good, ole time Buddahism (I tend to think that Tibetian Buddahism is a good replacement for Abrahamic religion).
Personally, I really don't think that the battle of Talas mattered
that much of a difference when it comes to the Chinese losing their influence in Central Asia, as the Tang Dynasty was already in decline at this point, and it is quite likely that the Tang would have lost their holdings in Central Asia anyway (...allthough not neccesarily to the Arabs).
As for Buddhism: Buddhism had already existed in Central Asia for a long time at this point.
In fact - there had been significant Buddhist communities in Central Asia long before Buddhism even got established in Tibet.
What's more; there was great religious diversity in Central Asia at the time - there were significant numbers of (predominately Nestorian) Christians, Buddhists, Manicheans, Zoroastrians (including heterodox Zoroastrian sects, such as the Mazdakites) Jews, and various indiginous religious traditions here.
And none of those religions will dominate the region, unless a powerful nation emerges, and imposes its official religion on the peoples that it conquered and subjected.
And Nestorian Christianity
did have a strong presence in Central Asia at the time, and there were quite a few Christian Turkish tribes and tribal confederations in Central Asia (a number of them lasted as long as the late Mongol period).
The rise of a Christian empire in Central Asia if Islam fails to gain a foothold there may not be inevitable, but it's certainly one of the more likely possebilities.