I can explain but there might be spoilers to The Brain of Morbius

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During the final confrontation between the Doctor and the Frankensteinish-resurrected Morbius, they both decided to do a type of mental duel, using some metal as a conduit for their mind battle. During the mind-contest, several faces mentally appear, some of them are the Doctor's known incarnations and the the Morbius monster, and some unknown faces.

Some viewers think of those unknown Doctor's as the Doctor's previous incarnations)

In addition to what has been said above, I believe some sources explicitly confirm that the Morbious Doctors are pre-First Doctor

As of The Timeless Children, they are confirmed to be incarnations of the Doctor prior to William Hartnell's incarnation. However, up to that point, since their appearance in The Brain of Morbius, there was much speculation as to who they actually were, some accepted the (now confirmed) theory that they were the Doctor, while others preferred the theory that they were the incarnations of Morbius.

The Morbius Doctors briefly appear in The Timeless Children, confirming that they are the Doctor.

Yeah, that is true, though there are some people who might not like it. And I originally thought they were Mobius incarnations too, to be honest. I would have preferred it the incarnations of the his predecessor The Other. That still is my favorite origin out of the other explanations, if it has to be the Doctor's