Randy Foye + Al Jefferson was the basis of our entire offense last year, along with Hayward playing off the ball. We don't have that at all this year because a) Foye is gone, b) Hayward is now the running around with the ball instead of spotting up, and then c) we don't have Al drawing double and triple teams in the post. FWIW, Marvin is a better wing defender and rebounder than Richard Jefferson, but that's a smaller factor.
I would say Kanter is a more effective offensive player than Millsap last year, given his FG%, pts. avg. and offensive rebound rate. Of the four players you mention--Al, Sap, Kanter, Favors, I would say Favors is the least effective offensive player and Al Jefferson is the most effective. Favors can only score in certain situations, like pick-and-roll and from 15-feet out. He's not getting those scoring opportunities due to congestion in the paint. Nonetheless, Favors should be scoring more against his defender than he is currently.
