If Matthews is healthy enough (you know 70 games or so), I think they actually have an outside shot at the playoffs as there are only 7 teams out west clearly better than them (GS, Mem, SAS, LAC, Hou, OKC, NO). I'd put them right there with the remaining 7 teams in the west (PHO, MIN, DEN, UTA, SAC, LAL, POR).
I mean they did win 50 games last year. Obviously losing Ellis and Chandler are fairly big blows (Aminu and Barea played key roles as well), but a healthy enough Matthews gives them much of what Ellis does. They acquired Pachulia, who is at least a credible big man (no where near as good as Chandler or Jordan, but not a total bum either). If they bring in Josh Smith, which has been rumored it adds to their interior defense. Williams would provide some decent shooting for like 40 games, but even without him they still have a decent back-court/wing rotation with Parsons, Matthews, Harris, Felton, and the rookie Anderson.
I wouldn't be surprised if Dallas finished 8th or they finished in dead last in the conference. Those teams all seem pretty close together in overall talent and fit.