The Argentine striker has impressed in bursts in the few matches he has played since joining the expensively assembled London side from Inter Milan for 16.8 million pounds ($26.28 million) in August.
In an interview on Friday, Crespo said that after a fitful three months trying to settle in London, twice interrupted with trips back home to South America for World Cup qualifiers, he could at last dedicate himself fully to Chelsea.
"Now I can relax because until March (Argentina's next qualifier) I can think only of Chelsea," he said.
He has struggled to show his best form, having missed the first three months of the year with a knee ligament injury, followed by holidays, the pre-season with Inter and then the change of club.
But he added: "In six matches (with Chelsea), and not all from the start, I've scored five goals. I can't complain and I'm still not at 100 per cent."
Suddenly finding his top form for Sunday may be a tall order, but Crespo said he would be helped by the relaxed atmosphere in the English game compared
"People (here) live it the way it should be experienced, as a spectacle, two teams giving their all, without things like: 'No, they're holding back, a draw suits them, they've come to an agreement.'
"It's seen as a sport, something nice to watch, a pastime, and that's brilliant."
"In Italy, if you're not 100 per cent, you won't make a difference," he said.
"Now I'll get into proper physical shape...on a par with the rest (of the players)."
Chelsea's attitude had helped enormously, he added, because manager Claudio Ranieri had not demanded that he and Argentina team mate Juan Sebastian Veron return immediately from international duty.
Veron will not be with Crespo against his former club. He has a nagging back problem that flared up again while with Argentina last week.
Crespo, who like Veron missed last weekend's 1-0 win at Southampton that kept Chelsea one point behind leaders Arsenal and one above third-placed United, said Sunday's match was a player's dream.
"I think it's one of those matches that you've got to play, that you love playing for what they mean," he said.