Well, I played through the Celes rescuing part just yesterday.
And I can say Lock always was fond of both.
When he first meets Terra, he swears he'l protect her, and he does the same with Celes. Plus, when she asks him why he's helping her, he answers "you remind me of someone I knew". So even if it is debatable if Lock liked Celes only because she reminded him of Rachel, indeed he did like her.