Steven Gerrard was "not shocked or surprised" that his Rangers side dropped points for the fourth time in seven Scottish Premiership matches after being held by St Johnstone.
The 2-2 draw at McDiarmid Park followed losses at Hearts and Kilmarnock and a draw with Aberdeen in 2020.
Having been within two points of leaders Celtic with a game in hand, the Ibrox side now trail by 12 points.
"I'm frustrated and concerned with us defensively," Gerrard said.
"It has happened too many times now. We keep shooting ourselves in the foot by conceding silly, silly goals and we forgot the real good, strong basics of defending."
Callum Hendry put St Johnstone in front after just seven minutes when Rangers centre-back Nikola Katic sliced his clearance into the striker's path.
And the Croat was also at fault for the equaliser, failing to get enough purchase on his clearance from a corner and allowing May to pounce with 11 minutes left.
Rangers had responded to lead when half-time substitute Florian Kamberi thumped home a volley, then set up Joe Aribo to score.
"We should be talking about those two players [Kamberi and Aribo] being superstars for Rangers today but unfortunately but everyone's going to criticise us on a poor defensive showing," Gerrard added.
"I'm going to have to work even harder than we do as a staff to find solutions and get back to the basics of defending. I need [Filip] Helander back fit sharp-ish."