You have the order wrong. It was Morris, who would be our starting PF, no questions, first. Then Carroll, who was to play some PF and some SF. When the Morris thing fell through, they signed Lyles off of the scrap heap.
I think what they're looking at now is "Can Lyles be an important piece with the New (post DD LMA) Spurs?". He's vastly improved in the last 3-4 games, and just turned 24 a few days ago. He's a former #12 overall pick. If he can come in, score 10-12 points, rebound enough for a PF, and shoot maybe 35-36% from 3, that would be a huge 'get'. He's obviously knowledgeable about the game, understands where he needs to be, and where everyone else will be.