Donati played alongside the German goalkeeper at Bundesliga club Mainz in 2016.
It was at the Opel Arena where Karius impressed enough to earn a move to Premier League giants Liverpool in the summer of 2016.
It was in the 2017/18 campaign where Karius went down in Liverpool folklore for all the wrong reasons.
The mistakes didn't stop there as the 26-year-old was to blame for two of Real's goals in the 3-1 victory for Zinedine Zidane's side.
Karius was sent to Besiktas on a two-year loan spell the following summer, which Donati believes was an overreaction.
He told Tribalfootball.com: "Unfortunately, the media and all the people around football chose to blow up on a mistake.
"But we haven't to forget that Liverpool reached that final even thanks to him.
"He was a young guy who had made a mistake in a final. It's true that it was a very important match, but it could happen and it shouldn't have deleted all the good things he had done.
"I'm sure he would have done well in the following years at Liverpool, but sadly we will not have the evidence now he's in Turkey."
Asked how he found playing in front of Karius at Mainz, Donati had nothing but positive things to say about his former teammate.
"Whenwhen I used to train with him and we played together, having someone like him behind you gave a lot of confidence: it was always something very positive."