The former Manchester United defender has been battling an ongoing knee issue for the past 18 months but is back in action for the Black Cats ahead of the 2013/14 season which is a huge boost says Johnson.
"We've got a world-class defender back," Johnson told BBC Newcastle.
"He's been the best new signing we've had. Even though he's not a new signing, it has felt that way.
"It shows the type of player he is. We've seen how composed and top class he is and if we keep him fit we'll definitely be a stronger team."