“Sacrifice as if [they were] present” means that, when sacrificing to the spirits, you should comport yourself as if the spirits were present.
The Master said, “If I am not fully present at the sacrifice, it is as if I did not sacrifice at all.”Confucius, & Slingerland, E. (2003). Analects: With selections from traditional commentaries. Hackett Publishing.
Sacrifice as if they were present means to sacrifice as if the gods were present. But the Master said, If I can’t take part in the sacrifices, it’s as though I haven’t sacrificed at all.Confucius, & Watson, B. (2007). The Analects of Confucius. Columbia University Press.