"They're still world champions and you have to take that into account," said Croft.
"South Africa have just lost a game they may have expected to win so they'll be on the rampage.
"If you start thinking South Africa are not quite the team they used to be, that's where you will definitely slip up.
"They're missing some good players, but they've got other good players with a huge amount of experience who can come in and play.
"You have to have respect for them off the pitch but as soon as you cross that whitewash, it's you versus them and if you give them that respect on the pitch, they will get the upper hand."
Those sentiments were echoed by Lions legend and England manager Martin Johnson who believes South Africa still present a huge challenge.
"They have lost one game on the tour here and won two," said Johnson.
"They're still a very good team, they're missing some good players, but they've got other good players with a huge amount of experience who can come in and play.
"Things happen very quickly, expectations change. A couple of weeks ago, on the Friday before we played Australia, everyone was talking about them as the best team in the world - and they had a point.
"By the Wednesday they'd lost two games and suddenly they were in crisis. What this autumn series has highlighted is any of these teams is capable of beating the other one."