The Reds boss coached the striker between 2001-2003 while in charge of Mainz.
‘An incredible talent’
Voronin would eventually move to Anfield on a free transfer in the summer of 2007 after spells with FC Koln and Bayer Leverkusen.
However, the former Ukraine international struggled on Merseyside.
Voronin netted just six goals in 40 games before he was shipped off to Dynamo Moscow three years later.
But Klopp believes the centre-forward's career was held back as he never fully believed in his ability.
Speaking to Mainz's Youtube channel, when asked who of his former players would he re-sign if he could, Klopp said: “Andriy Voronin.
“If Andriy had known just one day in his life how good he could have been, it would have been absolutely amazing.
“Andriy was an incredible player, an incredible talent.
“Actually, I was a little unfair. In my first year as coach, I think my first full season as coach especially, I would always only sub him on.
“And then Andriy came to my room during the training camp and asked: ‘Kloppo, why do I never start?’
“I said: ‘Do you want the honest answer?’ He said: ‘Yes.' So I said: ‘Because you don't complain when you don't start!’
“He replied: 'Fair enough!' And he carried on not starting.”
Where is Voronin now?
Voronin retired from football in 2014. He is now assistant coach at Dynamo Moscow.
They finished seventh in the Russian top flight last season.