Keba is a better shot blocker, rebounder, rim runner and overall defender, he moves better laterally
Fouss is much better with his back to the basket and can really get you a bucket when you need it. He is the better offensive player and decent at defense

Fouss shows better in the advanced metrics, so maybe he will start at some point. At the end of the day both will split time, with Fouss getting more minutes until Keba can get healthy.