Ejim isn't a big. He's been thrown into that position and he's playing his butt off, but he doesn't have the size to block the shots of or deter penetrators (which should be obvious from the way ISU is abused in the paint) or the size to finish on offense in a crowd of bigger players.
Last night, in OT, Ejim got the ball, put his defender on his back, and was two feet from the basket, with defenders closing in. He passed out for three that was missed. And I don't blame him for passing out, because his shot would have had a good chance of being blocked by the bigger defenders. With a true big, at a minimum, that play should have resulted in 2 FT attempts, and more likely a dunk and chance for a 3pt play. On another occasion in OT, Ejim was driving for a dunk and got blocked from behind by UT's center (who isn't exactly have jump-out-of-the-gym capability). Another time in OT, he got pushed to the floor fighting for position, which resulted in a TO.
Ejim is playing his heart out at a position that he doesn't have the size and strength for right now. I sincerely hope he is being forced to play post because of the issues with Booker and Gibson, and that Hoiberg doesn't expect a 6'6" 230 pound kid to enforce the post in the Big 12.