Cascarino was unimpressed by the Manchester United loanee in last week's draw with Nottingham Forest.
He wrote for The Times: "There were no big mistakes, everything was neat and tidy, he played short passes that were accurate.
"There was nothing to complain about, or that a pundit could pull apart in analysis on television.
"But he’s playing for Chelsea. This is a club that has high expectations of players, especially attacking ones. Being OK, which is what he was, is a problem.
"The truth is that he made it easy for Nottingham Forest’s right-back Ola Aina to play against him.
"There is no question Sancho is a good player, and his touch is almost always really sound, but he plays such a conservative game.
"He never got into a physical battle with Aina and never tried to take him on. Sancho is not slow but he doesn’t have that burst of acceleration to get at defenders."
He added: "Cristiano Ronaldo, Eden Hazard, Gareth Bale and the other best wingers of recent times at the biggest teams in the Premier League have always wanted to overwhelm the full-back marking them, using power and speed to beat them physically.
"Sancho needs to get a bit stronger, and try to develop so that he can have a more significant change of pace.
"That will allow him to keep a defender guessing – at the moment, the full-back knows there is very little chance of him trying to get to the byline."