On Saturday, Nair was dropped from India's Test squad for the home series against West Indies. He was left out after spending all five Tests on the England tour on the bench.
"This is a mystery that needs to be solved," Harbhajan, who worked as television commentator on the England tour, told PTI. "How can a person sitting on the bench for three months become so bad that he is not good enough to remain in the squad? Trust me, I am at pains to understand the thinking of this selection committee and the parameters that they are using for selecting national squads."
Nair made his Test debut in 2016 series against England, in which made an unbeaten 303 in the third Test in Chennai. Since then, he has played only three home Tests, against Australia, and none since March 2017. Nair has had a high score of 26 in four innings in Test cricket since his triple-century.
After the Australia series, Nair was dropped for the home and away series against Sri Lanka and for the Tests in South Africa. He was named for the one-off Test against Afghanistan, but did not make the playing XI. In July, he travelled with India A to England, where he made two fifties in six first-class innings against England Lions and West Indies A, with a top score of 93.
To compound the confusion over Nair's role in the squad, he was overlooked for the final Test against England this summer even when reinforcements were required; India handed a debut to Andhra batsman Hanuma Vihari, who was not a part of the original squad, at The Oval.
"I find different set of rules for different players. There are some who are given a long rope to succeed while for select others they don't even get one chance to fail. That is not fair," Harbhajan said. "What if Hanuma Vihari fails in two Test matches against West Indies? Though I would never wish something like that for any player. I wish him [Vihari] all the very best.
"But [if Vihari is not able to deliver] you will go back to Nair but will he be battle-ready with confidence for Australia tour [at the end of 2018]?"
Harbhajan also questioned whether the team management and the selection committee were on the same page. "If team management did not play Karun for five Tests, then it means that he couldn't inspire confidence in them.
"Well, my question is whether the selectors were able to convince the captain while naming Karun for England series? If not, did they seriously consider the name that team management had in mind. Hope all concerned with team selection undergo course correction before they decide on the [squad for the] Australia tour."