Spain's Fabregas attend a training session at Fortaleza city
The taciturn Spain coach, who keeps his cards closer to his chest than Cool Hand Luke in a poker game, said the pair would be "important casualties" if they were ruled out.
"I don't think we will change our system against Italy, but we will wait and see how Soldado is, and how Fabregas is doing before we have to make any decision," Del Bosque explained.
"If they don't make it, we have alternatives. We would miss them for sure, but I don't want to over dramatise it, and we have not yet made the decision."
One indication the pair could start was that both trained in the Estadio Castelao after the media conference with no obvious discomfort.
Fabregas came off in the 54th minute of Spain's 3-0 win over Nigeria in Fortaleza on Sunday, while Soldado limped out six minutes later with the Spanish FA confirming on Tuesday that both had muscular injuries which had been scanned.
Spain have not lost for 25 matches since being beaten in a friendly by England in Nov. 2011, but their unbeaten competitive run is even longer and stretches back 28 matches to the first game of the World Cup finals in South Africa in June 2010 when they surprisingly lost to Switzerland.
Del Bosque said there was no magic formula in establishing such a long unbeaten streak, saying: "There is no single reason. We all know football, we need good organisation, team spirit but a really competitive spirit too, that is essential."
The 62-year-old would not be drawn on his final team selection for the game against Italy, their first meeting since Spain beat them 4-0 in the final of Euro 2012 a year ago.
However, there are widespread reports that goalkeeper and captain Iker Casillas may return to the starting lineup to win his 147th cap.
Del Bosque rotated Casillas, Pepe Reina and Victor Valdes between the sticks for the group stage matches against Uruguay, Tahiti and Nigeria, and although he would not confirm that the Real Madrid custodian would play, his Barcelona rival Sergio Busquets, gave the game away instead.
"He is very important for us, he is the captain and he has played more matches for Spain than anyone else and it is really important to have him back on the field," Busquets said.
Casillas's team mate at Real, defender Alvaro Arbeloa, added it made no difference to Spain that Italy's most dangerous striker Mario Balotelli would be missing through injury.
"They have other players just equally effective," Arbeloa said.
"I don't think that will make much difference to us, or to them really."
Despite trouncing Italy in the Euro final last year, Del Bosque was still taking nothing for granted.
"They were superior to us when we played in the group match last year, lets not forget that. The two sides know so much about each other, there will be few surprises on Thursday but obviously we will be looking to get to the final and beat them again," he said.
The winners face either Brazil or Uruguay in the final in the Maracana on Sunday.
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