With Lukaku having opened the scoring for Inter midway through the first half, Getafe veteran Molina had the chance to restore parity with 14 minutes remaining following Diego Godin's handball.
Yet Molina fluffed his lines, hacking his effort embarrassingly wide just seven minutes after coming on as a substitute in Germany, where a one-off tie was played after the coronavirus pandemic delayed the original first leg.
A glaring miss from Lukaku moments later gave Getafe some hope, but Eriksen was on hand to extinguish any chance of a comeback and seal Inter's last-eight spot.
Samir Handanovic was called into action inside two minutes, diving to his left to keep out Nemanja Maksimovic's header, and Alessandro Bastoni's block denied Jaime Mata.
Lautaro Martinez forced David Soria into two fine saves in response, before Lukaku went one better in the 33rd minute, slamming home with a brilliant finish after latching onto Bastoni's pass.
Danilo D'Ambrosio and Godin each almost doubled Inter's lead following the restart, while Handanovic pulled off an excellent stop from Mata's header at the other end.
Getafe were handed a lifeline when the VAR spotted a handball from Godin, prompting referee Anthony Taylor to visit the pitchside monitor and then award a penalty.
But having just replaced Mata, Molina got his spot-kick horribly wrong, sending a terrible attempt a yard wide of the left-hand upright.
Lukaku should have finished things off moments later, only to send a close-range shot wide, but his miss mattered little when substitute Erisken struck from eight yards out to compound Getafe's misery.