Moise Fokou obviously isn't the reason Titans are losing, but they certainly don't have the same feel to them at the linebacker position.
Before the Fokou injury, teams were averaging 111.1 rushing yards per game and had only allowed five touchdowns over the course of six games.This is compared to the four games he has missed, where the Titans have allowed 126 rushing yards per game and a whopping 10 touchdowns.
This sort of drop off isn't the product of playing harder teams or better running backs, but an issue with taking down running backs on first contact. When Fokou has been in the game, the Titans have been an enforcing defense that doesn't allow teams to beat them with the running game and that is what they need to get back to.
In games where Moise Fokou has played the Titans have been very good rushing the passer. Whether they get there by stunting LBs or DBs, Gregg Williams seems to feel comfortable letting him take running back's one-on-one. However, since Fokou has been out there have been exactly ZERO linebacker sacks. That is another drop off that can't just be attributed to a change in opponent.
At the end of the day, I don't know why the Tennessee Titans have been so tentative on defense since the Fokou injury, but it has left them crippled and exposed some of their cracks. If the Titans want to get into the playoffs, then they need to get back to being aggressive and pray that when Fokou comes back he is as good as when he left.