This helps a bit: http://basketballvalue.com/player.ph...11-2012&id=812
You see he has a positive adjusted +/- rating. The On/off court defense differential is -2.72 (that's good - the team has a better defensive rating when he's on the floor). But this number is "unadjusted", so it doesn't take into account whether he was guarding elite players or bench players in garbage time. Since he started ~ 2/3rds the games last year, its reasonable to assume he largely played against above average wings. So, by the numbers, he is a good defender (the "eye test" confirms this as well).
This may also be helpful. It's a graph of Advanced Statistical Plus/Minus (ASPM) scores - both offense and defense.
As you can see, it shows Johnson as Toronto's best defender.