Of course you can. He just did.
He is a football coach at a university. He doesn't write laws, he didn't establish any religion.
At some point the state is endorsing a particular religion (here Christianity) when: a) the state pays the coach; b) the coach at a public university continually endorses a particular religion; and c) (hypothetically) non-Christians feel that they must either be Christian, or not play/coach/support football at the University.
The big step is (C), and I am not particularly worried about that occurring at UConn. BUT - there is simply not reason that a single atheist, muslim, jew, buddhist, nihilist, etc. should have to tolerate another religion being forced on them at a public institution and I think that is why a lot of people have a problem with the "Jesus" quotes.
And if I had to guess, I would assume that most of those that don't have a problem are Christian. And to you I ask, how would you feel if, in your career field, your boss had a required lunch every week where he made it clear that every success your organization has had, and will have, comes from the love of Allah. Would you feel awkward being Christian, or see your career limited unless you began to comport with his beliefs? Because that is exactly what you are asking a non-christian UConn football player to deal with.