I'm no romance writer (at least not yet), but I would ask if the MC knew how this character felt already, did the MC have feelings for him as well, and or were they already in a relationship before what ever incident caused this character to lose his memory. What made them fall for each other in the first place anyway? I for sure know there's a couple ways to do this, so here's just a couple ideas
-If the character was involved in some sort of accident and needed time to recover, the MC could aid them. It could make for a really awkward atmosphere in the beginning. The whole idea of a vaguely familiar stranger not wanting to leave your side is kinda sweet. If that for some reason turns your character off, it could really make for some interesting tension.
-They could both end up in a situation that has the MC expressing or doing whatever got the other character interested in the first place because somethings about your brain just don't change. It could really get the questions firing between the two.
hope any of this helps.