The former Liverpool boss did speak earlier this week about snubbing Sadio Mane due to his price-tag before eventually working with the Senegal legend at Anfield.
But Red Bull football chief Klopp expanded on the transfer calls he still regrets while in South Africa yesterday.
Klopp revealed: "Here is a Dortmund fan to whom I have to apologise. Because I did not sign Sadio Mané for BVB."
At that time, Mané was still playing for RB Salzburg, but the €41m transfer fee that the Reds paid in 2016 was "a lot of money for a player that I could have signed for much less a few years ago".
Klopp also recalled passing on Heung-min Son, the Tottenham captain, when he was still playing for Hamburg SV: "I could have brought Son to Dortmund. When I faced him later in the Premier League, I just thought: 'My God, you stupid idiot'."
In 2013 Son moved from HSV to Bayer Leverkusen, and in 2015 to Spurs for €30m.
Kevin De Bruyne, of Manchester City, was another who almost played in black and yellow.
Klopp said of the Belgian, who was still under contract with Wolfsburg at the time: "We were so close with him."
However, De Bruyne's market value increased rapidly in 2015, and he eventually moved to City for €76m.