Eriksen, who signed for the Serie A club in January, scored his first goal for Inter in Thursday's 2-0 Europa League last-32 win away at Ludogorets.
The Serie A side were not at their best at Ludogorets Arena on Thursday.
Ludogorets were at their best in defense. Plamen Iliev made some great saves, but yet the Seria A team outplayed them.
Inter, who began with Lukaku on the bench, struggled to impose themselves in the opening minutes.
The upright kept out Alexis Sanchez's clever flick after the restart, then Lautaro Martinez lashed a long-range strike narrowly over the crossbar as Inter increased the tempo.
Iliev made another fine save to turn away Eriksen's bouncing 20-yard volley, but the goalkeeper could do nothing about Inter's first goal in the 71st minute.
Ludogorets, who were without their star striker Claudiu Keseru due to injury, failed to record a shot on target.
Lukaku scored frompenalty in added time following Anicet Andrianantenaina's handball.