Spring was sent off for a dangerous tackle on Andrew Welsh in the 14th minute and lost captain Stephen Dawson to injury in a first half that saw Yeovil midfielder Shaun MacDonald help himself to three goals.
Welsh and Dean Bowditch were also on target for Yeovil, while Dean Cox scored a late consolation goal for Orient.
Slade said: "It was an absolute disaster for us, there is no point in saying anything else. Two decisions in the first place were absolutely key to an afternoon of huge disappointment.
"You just couldn't see it coming, it was unbelievable. The first decision is the sending-off of Matt Spring.
"I haven't seen it on the DVD yet but I have spoken to the player and he insists that it's not a red-card offence. We'll have to have a look at that and if we see fit, then obviously we'll take steps to appeal."
Source: PA
Source: PA