The Super Bowl champions could have up to seven new faces in their starting lineup for the 2013 NFL season.
The head coach told the Ravens' website: "Communication is the final piece. We've got a lot of communication issues on defense that came up, things we need to work on.
"From a cohesion standpoint, we have a lot of work to do. Penalties and mistakes. If we don't do that on defense, people are going to have a tough time moving the ball on us."
The Ravens beat the Tampa Bay Buccaneers in their opening pre-season game on Thursday.