Schalke shocked Inter with a 5-2 win at the San Siro stadium last Tuesday in the first-leg to put the Bundesliga side on the verge of a semi-final against either Manchester United or Chelsea.
Ex-Real Madrid star Raul, 33, who scored his 70th Champions League goal at Inter, says the Germans still need to concentrate to make sure they reach the last four.
Inter have to win at least 4-0 in Gelsenkirchen to stay in the competition and the Italians' star striker Samuel Eto'o and play-maker Wesley Sneijder will be looking to test Schalke's Germany goalkeeper Manuel Neuer.
"We need to keep our feet on the ground and show Inter respect, because they will be looking to turn it around in Germany," said Raul. "We'll have to perform well again to ensure we reach the last four."
Inter coach Leonardo has called Argentine defender Walter Samuel into his squad, only five months after a knee operation on his cruciate ligament, and Dutch star Sneijder is expected to play, despite suffering from the flu.
Milan's ex-Bayern Munich defender Lucio is available again after missing the first leg with suspension, while Schalke are without Peru striker Jefferson Farfan, who picked up his fifth yellow card at the San Siro last week.
Schalke have consistently saved their best form for Europe this season, as their placing of ninth in the German league testifies, but coach Ralf Rangnick says any talk of the semi-finals is premature.
"We are not in the semi-finals just yet, but we have a pretty big chance now," said Rangnick, who only took charge last month after Felix Magath was sacked.
"Had someone told us before the (first-leg) match that we would beat Inter by three goals, we would have had declared him crazy. We had a real chance and we took it, but now we have to focus."
Schalke warmed up for the Inter visit by beating relegation-threatened Wolfsburg 1-0 at their Veltins Arena stadium on Saturday, while Inter laboured to a 2-0 Serie A win over Chievo at the San Siro last weekend.
Inter's coach Leonardo insists the holders can still hope to progress against Schalke, despite needing to score four goals in Germany.
"For me, everything is possible. I think it and so do the players, all of them," he said.
Inter will need to a near-miracle to take their place in the semi-final, but Schalke's defence has been erratic on occasions this season and Neuer will need to be at his best to make sure the Royal Blues march into the last four.