First, take them at their word. Celebrate and rejoice that they’re not harbouring some sort of anger or dissent. Say, “oh, that’s great! I’m so glad to hear it!”
Second, verbalise your fear. Tell them that you were concerned they might feel something. Name it. Explain why it would be possible they could have felt that. Something like, “I have to admit I was a tad worried you might feel left out after I’ve asked Terry to start X and Trevor to start Y… I really hoped you weren’t going to feel there was anything personal.”
Third, ask again, but this time be more specific about the type of feeling, and when they might have felt it… “When I did those things, did you have any of those feelings?”
Generally, people will avoid telling you their feelings because they don’t think you could understand. So when you name their feelings, it’s much easier for them to say, “actually, yeah, I did feel a bit like that”.