Basler has labelled the club's decision to release the 31-year-old as incomprehensible and publicly apologised for doubting him eight years earlier.
Goretzka will leave Bayern Munich on a free transfer at the end of the season after the club confirmed in January that his contract would not be extended.
He has spent eight years at the club since joining from Schalke 04 in 2018, making 309 appearances, winning eight Bundesliga titles and a Champions League. AC Milan and Goretzka are understood to be close to agreeing a three-year deal worth around €5 million per year.
Basler, speaking on Sport1's Doppelpass programme ahead of the second leg — which Bayern must win to overturn a 5-4 first-leg defeat in Paris — was blunt about his frustration.
"I'm sorry he's leaving Bayern Munich. It pains me to see him leave. I find that incomprehensible. He has all the right qualities, both defensively and offensively. That's why I can't understand why they're letting him go."
He was also critical of Kompany's selection in the first leg, where Aleksandar Pavlović started alongside Joshua Kimmich in central midfield, with Goretzka unused until the closing stages.
"He is the more reliable option. He should start the second leg."
The admission carried extra weight given Basler's history with the player. In 2018, appearing on the same programme, he predicted that Goretzka would not play five games for Bayern after joining from Schalke. The midfielder made over 300. Basler did not shy away from the contradiction.
"I can't remember that anymore, but I'm allowed to be wrong once in ten years. I made that statement at the time and believed that the move from Schalke to Bayern was simply too big a step. It's brilliant that he's ultimately developed so well."
Goretzka himself reflected on the story with amusement in a farewell interview for Bayern's club magazine.
"Basler said I wouldn't even play five games for Bayern. It was a few more than that. I don't even know if Peter Neururer ever got his crate of beer."
Sporting director Max Eberl also paid tribute, describing Goretzka as a player who consistently responded to adversity and chose to stay through the winter despite approaches from Atlético de Madrid.
"When we told him 18 months ago that it would be tough, he played very well that season. Last summer, when we said it might be tough, he fought his way back."
Bayern trail PSG 5-4 on aggregate and require at least a two-goal winning margin at the Allianz Arena to progress to the final without extra time.