Like Grant, Di Matteo took charge of the Blues part-way through a season and led them to the Champions League final.
But while Grant's man were unlucky beaten by Manchester United on penalties in 2008, yesterday [Saturday] Di Matteo's Chelsea were on the right side of the spot-kick equation as they beat Bayern Munich to become European champions for the first time.
Grant told The Sun on Sunday: "He is the man. Roberto Di Matteo is responsible for the biggest achievement in Chelsea's history.
"What he has done is unbelievable. He has led the team out of a miserable season to win the Champions League and the FA Cup."
Di Matteo must now wait to discover if he is given the manager's job on a permanent basis.
"I'm not the guy who makes the decisions at Chelsea but he's the man," said Grant.
"Sometimes when a club takes on a coach who is not so famous and the team is not doing so well everybody would say it's his fault.
"So when it goes well, he should get the credit. It's justice."
Chelsea's much-questioned older stars were to the fore last night, with Didier Drogba scoring scoring a late equaliser to send the game into extra time and the winning penalty, while Frank Lampard also impressed.
And Grant believes the victory shows previous manager Andre Villas-Boas made a mistake in trying to phase them out.
"Chelsea only got to the Champions League final because of the players everyone calls 'old'," said Grant.
"Look, Andre Villas-Boas started to play without them and what happened to results?
"Chelsea scored with their first corner of the match and who scored it? Didier Drogba.
"Frank Lampard loves getting into the box, he loves scoring goals but when Bayern Munich were dominating what did he do?
"He dropped back and played more defensively. How many balls did he win in midfield and play forward? Lampard followed orders to play for the team.
"So you can go on and on about getting rid of the old guard but this season and next season, whoever is manager will need experienced players - and, at Stamford Bridge, those players are more important than at any other club."