Say what you want about Cal/UK/ the DDMO not needing a true center, but there still has to be somebody on the floor who does some of the things a true center does. Mainly, guard the interior, snag defensive rebounds, and offer at least something in terms of a low post scoring option.
Looking ahead to next year, making the (seemingly safe) assumption that Enes Kanter is not a part of the equation, and Terrence Jones will also be gone, I would not be surprised to see Kyle Wiltjer starting. No one seems to be talking about him, as he's clearly the least heralded of UK's 4 signees. But, I think Cal took him because of how well he could potentially fit with the others, particularly how he could fit with Michael Gilchrist and Anthony Davis.
Gilchrist is a college small forward, but he's a college small forward who spent the first three seasons of his high school career playing (and dominating) in the lane, against some elite level competition. It wasn't until this year that St. Patrick's has had any players with real size inside to help Gilchrist. Also, Gilchrist appears to have grown some, and might be close to 6-8. So you have a guy who has all the ball skills of a small forward (handles, passes, is a threat to score on the drive or off a jumper), but also contributes inside like a power forward. In the FIBA Under 17 championships, Gilchrist was second on the team in rebounding, second in scoring (on a ridiculous 65% shooting), and in the only two even remotely close games the US played, went for a combined 54 points and 30 rebounds.
Davis has famously grown to 6'10", with insanely long arms, after being a guard most of his life. He has maintained his perimeter skills, while becoming a dominating rebounding and shot-blocking presence based on his extreme length. He still seems hesitant to play inside offensively, though.
Wiltjer is a 6-9 kid known for having a versatile offensive game. He has a fantastic looking jump shot, but also has a rep for showing some nifty old-school low-post moves. His athleticism, and thus his ability to defend, are questionable.
Now, none of these three, including Wiltjer, is really anything like a center. However, if you put them on the court at the same time, you have every facet of an interior game covered pretty well, at least in theory. If Wiltjer can't defend the lane, then Davis and Gilchrist can. If Davis can't score from the blocks, Wiltjer and Gilchrist can. Wiltjer handles the bulk of the "center" offensive responsibility, Davis the defensive. Gilchrist contributes to the overall interior play.
The really big question mark is physicality. Gilchrist is slender (though definitely wiry strong), Davis is rail-thin, and Wiltjer weighs no more than 230 lbs. In terms of skill set, they could have it all, but it won't work if they're consistently losing physical battles to older, stronger players.
Lots of well-informed, thoughtful people here, so I'm curious to hear what others think.