Lippi was due to reduce his 30-man squad to the required list of 23 but has instead cut just Grosso and Juventus team-mate Antonio Candreva and will wait until nearer the June 1 deadline to make a final decision.
"I don't do an Italy with debts of gratitude," Lippi told a news conference. "28? I have given myself 10 more days to reflect."
The Azzurri face Paraguay, New Zealand and Slovakia in Group F at the June 11-July 11 tournament in South Africa.
Left back Grosso has been in poor form for Juventus, who finished a miserable seventh in Serie A this term, but his exclusion was still a surprise given Lippi has tried to base his group on the 2006 World Cup winners.
Genoa's Domenico Criscito now looks set to play left back despite limited international experience.
Midfielder Candreva was always on the fringes of the squad.
Lippi made no direct comment about his future but again hinted he will step down after the World Cup, just like he did four years ago, with the Italian football federation promising a final decision before the team depart for South Africa.
Italy head for altitude training on Sunday in the Alpine ski resort of Sestriere in a bid to get used to South African conditions and Lippi now has extra time to run the rule over his strikers with perhaps two needing to be cut.
He decided against recalling injury-prone AS Roma forward Francesco Totti, officially retired from international football, when he named his 30-man squad last week.
"We all know the difficulty of Totti playing one game every three days," Lippi added.
Squad:
Goalkeepers: Gianluigi Buffon (Juventus), Morgan De Sanctis (Napoli), Federico Marchetti (Cagliari), Salvatore Sirigu (Palermo)
Defenders: Salvatore Bocchetti (Genoa), Leonardo Bonucci (Bari), Fabio Cannavaro (Juventus), Mattia Cassani (Palermo), Giorgio Chiellini (Juventus), Domenico Criscito (Genoa), Christian Maggio (Napoli), Gianluca Zambrotta (AC Milan)
Midfielders: Mauro Camoranesi (Juventus), Andrea Cossu (Cagliari), Daniele De Rossi (AS Roma), Gennaro Gattuso (AC Milan), Claudio Marchisio (Juventus), Riccardo Montolivo (Fiorentina), Angelo Palombo (Sampdoria), Simone Pepe (Udinese), Andrea Pirlo (AC Milan)
Forwards: Marco Borriello (AC Milan), Antonio Di Natale (Udinese), Alberto Gilardino (Fiorentina), Vincenzo Iaquinta (Juventus), Giampaolo Pazzini (Sampdoria), Fabio Quagliarella (Napoli), Giuseppe Rossi (Villarreal)
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