As far as Vedic tradition is concerned, Guru, Acharya, Upadhyaya etc., are all required to have some specific qualifications. Like those given below:
Manu Smriti 2.140. They call that Brahmana who initiates a pupil
and teaches him the Veda together with the Kalpa and the Rahasyas, the
teacher (acharya, of the latter).
2.141. But he who for his livelihood ...
The eminent Indus Valley Script expert Iravatham Mahadevan accepted Harvard's Michael Witzel as his guru,
Iravatham Mahadevan (11.15 to 11.25 a.m): “Guru Brahma, Guru Vishnu, Gurudevo Maheswara; Guru sakshat Parabrahma tasmai sri kuruve Namaha” ...