Over the last month, Courtney Lee learned a lot about keeping his chin up.

He’s due to be paid roughly $16.4 million over the next three seasons by the Celtics, but there’s a difference between financial security and rotational security.

Right when the Celtics seemingly could have used his defense — as New York’s Raymond Felton was confounding every defender thrown at him — Lee was dropped from the playoff rotation.

After Lee played 20 minutes and scored only four points in Game 1 of the Celtics’ Eastern Conference quarterfinal series against the Knicks, coach Doc Rivers announced he was consigning the guard to the end of the rotation, and worse.

He sat out the Celtics’ wins in Games 4 and 5, and played an average of 6.3 minutes in Games 2, 3 and 6.

It’s admittedly an experience Lee has never previously encountered.

“The key word here is experience,” said Lee, his jaw slightly clenched. “It’s one that I’ve never been a part of before. But that’s where I am right now. It’s a matter of learning how to carry myself and learn how to be professional.”

Rivers had said Lee was handling his demotion well, before adding, “as far as I know.”

The truth is that Lee, beyond his veneer of being a good teammate, was chafing at the idea that late-season additions like Terrence Williams and Jordan Crawford suddenly had moved up the ladder at his expense.