Montanier's men slipped to a third consecutive 2-0 defeat against their Brittany rivals on Saturday, a result that means Rennes' 43-year wait for a major trophy goes on.
Former Real Sociedad boss Montanier was left battled by Rennes' display, and is hopeful that the disappointment of missing out on silverware will not affect their fight for Ligue 1 survival.
"It is difficult because I do not really have any rational explanation," Montanier said.
"We came here to play a great event. We knew what to expect, we knew our opponents who are, for us, difficult to face, but I have no rational explanation.
"Our job is made of cruel disappointments and defeats are hard but we should bounce back.
"We must recover quickly. it is a defeat that hurts but we must regroup."
