The first leg of the Europa League play-off kicks off on Thursday at 20.00 GMT in Portugal.
Immobile has not even made the trip after coming down with a high fever last night.
“We will nonetheless play with 11 men, just as we have on other occasions without Immobile,” assured the coach in his press conference.
“We’ll find solutions. I don’t know if he will recover in time for Udinese, we’ll see how it goes, but even not in ideal condition, he’s still good to have around.
“FC Porto are a very strong side, well-organised and fresh. They have some experienced players and other interesting youngsters, like Vitinho and Vieira. They only lost twice this season, so that says it all.”
Lazio haven’t exactly given their best in the Europa League this season and Sarri does not disguise it is hardly his top priority, even for a man who lifted this Cup with Chelsea.
“It is a very tiring competition. You finish at 23.00 on a Thursday and no longer even have the option of postponing your weekend game to Monday.
“Obviously, Chelsea were built to win the Europa League with 30 players all on the same level. This is a European stage for Lazio and we want to have our say, even if there are stronger teams than us in the tournament.
“It is always prestigious for the club to play at this level.”
The coach is also no fan of the new rule abolishing away goals in UEFA knockout fixtures.
“This new ruling will take many games to extra time, even though we don’t have a fixture list where anyone can really afford to play extra time midweek.”
Sarri has been increasingly frustrated with his Lazio players for their lurches between victories and total wash-outs, such as the 4-0 Coppa Italia quarter-final defeat to Milan last week.
In explaining his position, he made a reference to his Napoli side.
“I always expect a team with the right attitude, but it’s difficult to get into the minds of your players. I remember once my side lost the first Champions League game, the day after I was furious, but two players told me they were only focused on Serie A. Sometimes, they have this unconscious bias.”