I’m very sure they aren’t lovers, just someone who had a connection and deeply affected the professer in a platonic way. Also he doesn’t appear again in any of the upcoming chapters
(spoiler)
nothing is sure yet as we don't even get to know his name 201 chapters in, but from what I've understood no he wasn't in love with thw manager, but he was his first true friend I guess. Sometimes the silhouette of the manager overlaps that of Garam while they have sex in the future, but I think it's just Heejae's intrusive thoughts and he didn't actually want to be with the manager romantically
does anyone have any intel/spoilers on heejaes former manager? was heejae in love with him? i thought that was quite obvious, but i saw someone mention that they weren't lovers