There are a few angles to think about with this kind of statement.
First, getting drafted does not guarantee anything. Players get drafted and bust or flame out all the time. Very few live up to their hype. It actually is hard to be a long-term NBA player. So in that sense, basically everyone on the Celtics has proven they have a place in the NBA, which is more than most players trying to get into the NBA from college.
Second, most "average" NBA players actually had some draft buzz and were drafted somewhere in a draft. So those players have been in the place that many Kentucky players are now.
Third, there are countless examples of success in college NOT translating to the NBA, so these exercises always overrate college players based on how they look against other kids.
Fourth, these college players are nowhere near their prime.
Fifth, look at the rookie seasons of a lot of very good players...not always that great. And that's after 1 year of growth and a full NBA training camp, so if they jumped straight from college into an NBA game, it'd look even worse than their rookie year stats!
But let's go with it. Let's start with the Celtics, then we'll do the Draft Express Mock for the KU players:
Starting:
PG: Turner
SG: Bradley
SF: Green
PF: Sully
C: Zeller
Bench:
F/C: Olynyk
F: Bass
G: Smart
F/C: Wright
Wing: Craeder
G: Thornton
Then let's look at their stats this year, and their last year of college, as well as their draft position+year.
So by my count, we have players who were good enough in college to be drafted at: 2, 5, 6, 8, 13, 17, 19, 20, 21, 33, 34, 43.
Hopping on over to the draft express mock, I see KU players at:
3, 4, 24, 27, 36, 42
So that's looking a bit less overpowering, since we know none of the Celtics were full "flameouts," but that is still unknown about the KU guys, especially at picks 24+. And that's just 6 players! KU relies on Booker, Lyles, Ulis and Lee as well, who are freshman or sophomores who arent even projected to be drafted this year!
Then take a look at KU's players stats this year, and compare them to the C's from each of their last college year:
Point is, if the current (bad) C's roster all magically were changed to be the same age and were on the same college team, it would be a ridiculous college team. Add to that natural growth, nba coaching, and the fact they've all at least stuck in the NBA and its no contest IMO.
But let's keep going.
KU's roster is impressive this year, no doubt. So let's just look back at the last 4 drafts for the NBA quality equivalent, and look at those players' rookie year (i.e just out of college) PPG/RPG/APG/BPG/SPG just for quick reference:
2011:
Kanter 4.6/4.2/.1/.3/.3
Tristan THompson 8.2/6.5/.5/.5/1.0
Reggie Jackson 3.1/1.2/1.6/.6/0
JaJuan Johnson 3.2/1.6/.2/.1/.4
Jordan Williams 4.6/3.6/.3/.5/.3
Davis Bertans None
2012 (and i'm not wasting time writing out scrub rookie lines now, you get the idea)
Beal 13.9/3.8/2.4/.9/.5
Waiters 13.7/2.4/3/1/.3
Jared Cunningham
Arnett Moultrie
Orlando Johnson
Doron Lamb
2013
Otto Porter 2.0/1.0/1.5
Cody Zeller 6/4.3/1.1/.5/.5
Tim Hardaway 10.2/1.5/.8
Rudy Gobert 2.3/3.4
Ray McCallum
Pierre Jackson
2014:
Embiid
Gordon 5.8/3/.6
Napier 5.7/2.3/1.9
Bogdan Bogdanovic
John Obryant III
Nick Johnson
For fun we can keep going back:
2010
Favors 6.8/5.3
Wes Johnson
Damion James
Jordan Crawford
Terico White
Desean Butler
2009
Harden 9.9/3.2/1.8
Tyrek evans 20.1/5.3/5.8
Byron mullens
Demearre Carrol
Sam Young 7.4/2.5/.7
Patrick Beverly 5.6/2.7/2.9
2008 (actually a good team! but as rookies? Mostly not ready at all.)
Mayo 18.5/3.8/3.2
Westbrook 15.3/4.9/5.3
Ibaka 6.3/5.4/1.3blk
Darrell Arthur 5.6/4.6
Asik 2.8/3.7
Sean Singletary
So that's about what you can expect from those types of picks. It's amazing how not-ready a lot of very good players are their rookie year. Sobering.
Finally lets look back at actual best NCAA teams. National champs, which ones could beat an NBA team as rookies?
'14 connecticut
Shabazz Napier
DeAndre Daniels
Ryan Boatright
Niels Giffey
Lasan Kromah
Amida Brimah
Omar Calhoun
Phillip Nolan
Terrence Samuel
'13 Louisville
Russ Smith
Peyton Siva
Chane Behanan
Gorgui Dieng
Luke Hancock
Wayne Blackshear
Montrezl Harrell
Kevin Ware
Stephan Van Treese
Zach Price
'12 Kentucky
Anthony Davis (13.5/8.2/1 as a rookie NBA)
Doron Lamb
Terrence Jones
Michael Kidd-Gilchrist
Marquis Teague
Darius Miller
Kyle Wiltjer
Sam Malone
'11 Uconn
Kemba Walker
Jeremy Lamb
Alex Oriakhi
Shabazz Napier
Roscoe Smith
Jamal Coombs-McDaniel
Charles Okwandu
Niels Giffey
Donnell Beverly
Tyler Olander
Enosch Wolf
'10 Duke
Jon Scheyer
Kyle Singler
Nolan Smith
Brian Zoubek
Miles Plumlee
Lance Thomas
Andre Dawkins
Mason Plumlee
Olek Czyz
Ryan Kelly
'09 UNC
Tyler Hansbrough
Ty Lawson
Wayne Ellington
Danny Green
Deon Thompson
Ed Davis
William Graves
Tyler Zeller
Bobby Frasor
Larry Drew
Marcus Ginyard
J.B. Tanner
Patrick Moody
So really, none of these great college teams had through and through success, let alone as rookies. Even great prospects take at least their rookie year to get acclimated, and that's after a full training camp, let alone stepping in against NBA guys right now. Same thing if you look at the predicted draft slots of the KU guys currently. Just not consistent enough to beat an NBA team, even a bad one, in a 7 game series.