I posted some trade ideas earlier in the thread...
For the PF spot I like Onyeka Okongwu. Atlanta has him 8th on their depth chart but I think he would consistently be top 10 in rebounds and blocked shots with starters minutes. He's also a good screen setter and roller/lob threat and can shoot the mid-range. My hope is that they trade for Siakam and see Okongwu is expendable since he already signed a 4yr extension that averages around $15 million a year.
Xavier Tillman on the Grizzlies is on an expiring rookie deal and is only getting limited minutes so he could be a cheap guy to take a chance on. He's not going to be a scorer and he would not help our FT woes at all but he's one of only a handful of guys who can guard wings on the perimeter and also defend the area near the basket.
At the SG spot I like De'Anthony Melton. Philly is one of the top 3 teams in the East this year and he's moved into the starting spot after the Harden trade so I don't know that he's gettable mid-season but he's also a free agent so if Philly flames out in the playoffs and wants to go in a different direction with all their cap space, we could swoop in. We'd probably have to clear some salary first though. He's developed into a pretty good shooter (37-40% from three the last 4 years). On offense he plays a little like Malik Monk with some shot creation skills and a pretty good catch and shoot or pull up jumper. On defense he's one of the best in he league at generating steals and deflections in the passing lanes and he's also a solid on-ball defender.
There's also Matisse Thybulle who is limited to a catch and shoot roll on offense but is one of the elite players in the league at contesting jumpers without fouling. He signed a 3 year deal for $11 million per season last summer but Portland is a last place team so there's likely to be a lot of roster turnover up there.
Like most people I'm looking at the Bulls imploding and wondering if Alex Caruso gets moved as part of a rebuilding effort there. He's elite at generating on-ball pressure and also has pretty good size to match up with either guards or wings.
Marcus Smart is still one of the best defenders in the league and he hasn't gotten off to the greatest start with Memphis. He certainly would fit the bill as a tone-setter on the defensive side though I wonder given Davion Mitchell's shrinking role if he would be a poor fit here too.
I'm sure there's a lot of guys I don't know about around the league who are under the radar defensive specialists. It's still really hard to measure defensive performance statistically and I'm not paying for any of the pay wall websites with their own proprietary advanced stats, but those are the first names that come to mind for me.