everything in the NT is foreshadowed in the old -- but just because it's 'there' doesn't mean we'd recognize it when we read it. it takes great wisdom -- and ((well as Paul said)) they are mysteries that the prophets diligently searched for and angels longed to look into, but were not revealed. i mean now i can look back at the book of Judges and see the gospel written all over it, because the Spirit has opened my eyes to see Christ in all those things. but before these things were 'opened' to us, even while they weren't technically "new" they couldn't be discerned. they still can't without the Spirit -- the Spirit who was at work in Paul when he expounded on all those things. he quotes the OT all the time, and shows how the gospel is in it. for example in Galatians when he says the Hagar & Sarah are two covenants, Sarah the free woman, the Jerusalem above, and Hagar the slave woman, the covenant of the Law. that's not "new" it's from Genesis! but who understood it when they read Genesis, before Christ came? before He sent the Spirit to teach us?
Paul is showing us those things in the scripture in his letters, then they are surely worth reading.