On the A320/1, not too critial. The CDL allows one or all of them to be missing with various penalties. For example, the 320 has a penalty of about 1.3 tonnes to the RTOW, 5kts to be added to the VApp and LDR increased by 8%.
The 320 has them on one side of the cowling, the 321 both. Flying a 321 with one missing isn't that unusual if a 320 cowling has been fitted as a temporary measure.