Everything about this sucks, tbh.
It sucks that Morris reneged when he could have been a help on a dark horse run for the team. It sucks that the team traded Bertans for ultimately no reason (which makes other trades more difficult to pull off, too).
With that out of the way, here's an interesting article from the Nuggets' SBNation site that indicates that Lyles actually had a positive impact on the floor...when he wasn't sharing it with Jokic.
https://www.denverstiffs.com/2018/12/27/18156786/the-denver-nuggets-have-a-big-decision-looming-with-trey-lyles
If you don't feel like reading it, the long and short is that Lyles is effective when he has plenty of touches and can play an active facilitator role, and ineffective when he's asked to be a cutter or spot-up shooter off the ball. Seeing as the Spurs don't have a guy like Jokic on the roster, and that they don't really play the way Denver does (give it to Jokic and have everybody else move freely and quickly), he may be a decent 3rd big, which is something that the Spurs needed to address (lower priority than a 3/4 combo, but one nonetheless).
He doesn't address the big wing need, and that's the most bitter pill to swallow in all of this. Unfortunately, Morris was really the only guy left that did address that on the FA market, so, when he ed out, there was nothing to be done.