Seedorf left Milan in 2012 after a decade at the club, following a bust-up with Allegri that saw his first-team opportunities significantly reduced under the 46-year-old.
The former Netherlands international made more than 300 league appearances for the Serie A club from 2002 to 2012 and was part of the sides that won the UEFA Champions League twice, as well as lifting two Serie A titles.
"The reason I left Milan, without a doubt, was because of their current coach Allegri," he told Brazilian TV station SportTv.
"The rapport with him was not the best, as we didn’t get along.
"Milan are changing, (they) have less quality and it will take time for them to get back to the top."
"From the outside it might've looked as if I was not 100 per cent fit, but at my age you have to keep playing regularly to maintain your muscles," he added.
"Examples are Paul Scholes and Ryan Giggs (at Manchester United), who continued playing, but Allegri wouldn’t allow me to."
