I'm not sure those three rookies were the ONLY CHANCE at making our D something special. I think another year in the system, the increased communication, Rex and Rob tweaking the system will be dividends this year.
About the second part, not sure what you expect them to do. Go draft another three players? Kidnap three pro-bowlers from other teams? You fill your injury slots with the next best player you can, whether that guy is on your roster, or just out there. It's not ideal, but someone has be be better than no one!