In Star Wars: The Last Jedi, Rey looked for Luke Skywalker to help train her to be a Jedi. She found a much different Luke than the one who defeated the Emperor at the end of Star Wars: Return of the Jedi. This Luke experienced a disillusionment about the Jedi Order and was reluctant to train Rey, and that's thanks to his traumatic experience training a padawan of extreme power. It was assumed the padawan to which he referred to was Kylo Ren; however, a Reddit theory suggests Luke was talking about Grogu.
The theory came up in a larger Reddit thread about why Luke would train Grogu. Afterall, Ahsoka told Din Djarin that she sensed a great fear in Grogu, and fear's one of the root emotions that can lead a Jedi to the Dark Side; therefore, uncontrolled and substantial fear is a big red flag. Because of this, the thread asked why Luke would train a fearful Grogu when he was tempted to murder his own nephew as a result of dreams alluding to the Dark Side. The response was that Grogu's actually the pupil that prompted Luke's line, "I've seen this raw strength only once before. It didn't scare me enough then. It does now."
Grogu's strong with the Force, as seen by the skills he demonstrated during his time traveling with the Mandalorian and because Ahsoka and Luke acknowledged his strength plainly. It could reasonably be true that Grogu's power grew into something frightening because Luke underestimated his raw strength thanks to his limited experience with training others, as well as his limited contact with other Jedi.
It's telling that Ahsoka chose not to train Grogu, yet Luke took him on without hesitation. Ahsoka spent a significant time of her life dedicated to the Jedi Order, so she has seen the very best and very worst of the Force. She also had more experience with the Jedi than Luke. Luke was rebuilding something he was not previously part of, whereas Ahsoka walked away from that very way of life after her personal experience with it.
If Luke was referring to Grogu in The Last Jedi, that would explain why Luke looked scared and impulsively reacted to the Dark Side dreams he saw within Ben. Luke told Rey that he wasn't scared enough the first time he had witnessed a level of power similar to hers, yet in all the flashbacks to when Luke nearly killed Ben, Luke was afraid and concerned about Ben. Therefore, it makes sense that Luke was not actually referring to that moment.
However, the real doubt on the validity of this theory comes from the fact that there's no evidence that there was any deliberate connection between The Last Jedi and The Mandalorian while Rian Johnson was working on Episode VIII. The Last Jedi was released in December 2017, and Jon Favreau was announced as the writer and executive producer on The Mandalorian in March of 2018. While it's possible that line in The Last Jedi influenced The Mandalorian, The Mandalorian could not influence The Last Jedi.
The theory that Luke was actually reluctant to train Rey because of his experience failing to train Grogu works retrospectively when looking back on the series; however, there is no evidence that the stories were plotted out that way deliberately. Despite the fact that the theory does not seem to be true, it would be cool if it was, and until Grogu's future with Luke is explored in canon, fans have the ability to theorize however they choose.