Grayson's side, who let a 2-0 lead slip at Leicester on Boxing Day, were denied a late penalty before Leeds defender Andy O'Brien's stoppage-time own goal - his second of the match - gifted the visitors a point.
Goals from Max Gradel, Jonny Howson and Bradley Johnson had twice given Leeds a two-goal cushion, but David Nugent and O'Brien's two own goals in the second half hauled Pompey level, while both manager agreed Webb made a series of misjudgements throughout the game.
Grayson said: "It's frustrating that we were 2-0 and then 3-1 up and still didn't win the game. We were two-up at Leicester on Sunday as well before drawing, so we could have had six points and have come out with two.
"I was pleased with the players' work ethic and that we extended our unbeaten run to 11 games, but we made individual mistakes again. Kasper Schmeichel and Andy O'Brien had a breakdown in communication for the late equaliser, but they had done well earlier in the game.
"Ross McCormack had a stonewall penalty turned down. I asked the referee why he hadn't given it and he just said it wasn't a penalty. It was a big decision. I thought the referee was very average for both teams and that's putting it mildly."
Source: PA
Source: PA