The now-retired striker's old club go into the game at Camp Nou with a four-point advantage in hand over their Catalan arrivals, meaning Jose Mourinho's men will still be on top of the table regardless of the result.
However, Morientes feels that the balance of power will shift dramatically in favour of the Blaugrana if they triumph on Saturday night given they were 10 points adrift as recently as last month.
"If Barca win they will fill Madrid with fear in their bodies, because in just a short time they will have seen their 10-point cushion [dramatically] reduced," the former Spain international is quoted as saying by Cadena Cope.
"The psychological aspect will be a determining factor in the game. Madrid cannot play for a draw. They have to go to win, to attack and play the football that their players are capable of.
"If the game gets difficult, they can start thinking about just getting a draw. However, there is a lot of risk attached to just thinking about trying to defend against a team like Barcelona."
The top-of-the-table clash in Catalunya gets under way at 20.00CET.
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