The phrase "acceptable substitute" means another option or choice that is considered good enough or suitable when the original or preferred option is not available or suitable. Full definition
This recipe shows that beans make a perfectly acceptable substitute for meat. (vegkitchen.com)
I'm not one to roll out dough so this is a perfectly acceptable substitute. (elanaspantry.com)
But for romance, where the most price sensitive purchasers were, the lowered indie prices presented a perfectly acceptable substitute. (dearauthor.com)