Albion defended resolutely throughout, restricting the hosts to few goal-scoring chances.
The Seagulls occasionally posed a threat, themselves, on the counter attack, with the offside flag denying Andi Zeqiri his first goal for the club.
After Cengiz Under also saw his goal ruled out for offside, it looked like the tie was heading for extra-time.
However, Brighton were dealt a cruel blow with just seconds remaining, when Iheanacho turned in Youri Tielemans' cross.
Scroll down and click through to see how journalism student Sam Cox rated Graham Potter's men out of ten.
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After Cengiz Under also saw his goal ruled out for offside, it looked like the tie was heading for extra-time.