In the Bureau Sport interview, Boogerd says he thinks it's been a while since Dumoulin has displayed the form that led him to victory at the 2017 Giro d'Italia and the highest GC placing (2nd - 2018) by a Dutchman at the Tour de France since Boogerd himself finished fifth in 1998.
“I think Tom has peaked," Boogerd said as translated by Eurosport. "He has had some wonderful years. I hope he will return to his old level.
“But with everything he has experienced. Whenever I hear or see him, I think he looks very different from when he won the Giro."
“Then [when he won the Giro] everything was new, he was very spontaneous and cheerful. Now he’s agitated. That is also part of it, but it is not the Dumoulin who won beautiful races or had good results in his heyday.
“You will get to this point in your career, but it has gone completely wrong up to now. Before that it was all good and everything went well. Everything was a gift.
"He also rode without stress and without pressure. When you see him, you see a nervous and tense person. I recognise that because that’s also been me in the past.”
Boogerd speaks of Dumoulin's 'heyday' but the 29-year-old also finished runner-up at the Giro d'Italia just two years ago, the same year he also finished second at the Tour de France.
Jumbo-Visma's new signing missed most of last season and the early part of 2020 due to injury and illness.
While Boogerd's results from 1998 still stand, in 2016 his placings from 2005-2007 were voided after his 2013 Dutch version of the Lance/Oprah interview aired where he confessed to using EPO, blood transfusions and cortisone between 1997 to 2007.
Boogerd's confession followed years of denial after former Austrian cyclist Bernhard Kohl's manager implicated him in 2008.
In the Bureau Sport interview, Boogerd said he believes the men's peloton is still messing around with cortisone and also discussed the ESPN Lance Armstrong documentary series.