Warburton won two Six Nations titles, including a Grand Slam, with a Wales team built on the rock-solid defensive foundation provided by Shaun Edwards, who now holds that role for France.
The former Wales captain said Tandy needs help with defence.
“Steve’s admitted he’s probably spread himself a bit too thinly,” Warburton told BBC Sport.
“He’s been doing the defence with Dan Lydiate but then you’re trying to do the defence with all the other things that come with being a head coach.
“Dan is more the micro defensive skills [such as] tackle technique, competition on the floor.
“Wales don’t have a full-time defence coach [and] it’s not just about being a coach during campaign time. When the players are playing for their clubs, you’re there full-time, watching the regions play, or the Welsh-qualified players playing in England, and you can still keep dialogue and send clips.
“He needs to bring in a defence coach [but] it sounds like they can’t get people out of their contracts because it’s difficult to bring people in mid-season.
“For someone to come in as a full-time defence coach, they need to be tried and tested at this level.”
