Spain have three warm-up friendlies before the tournament. They take on Bosnia in St Gallen, Switzerland, on Sunday May 29, South Korea in Salzburg, Austria, on Wednesday June 1 and Georgia in Getafe, Spain, on Tuesday June 7.
"If we had called others up, you'd be asking about people who are now on the list," he said. "You cannot bring everyone. Cazorla's situation is very like that with Diego -- he has been out five months, made a great effort to be ready, but we cannot have doubts over too many people."
Chelsea boss Jose Mourinho confirmed after his team's 6-3 victory over Everton that Torres had left the club to join Milan on a two-season loan.
Although the players are due to meet up on Monday May 23, Del Bosque will be without many key players during the build-up as many members of the squad will be featuring in the finals of the Copa del Rey, FA Cup, Coppa Italia and DFB Pokal the previous weekend, as well as the Champions League final between Atletico Madrid and Real Madrid on May 28.
Del Bosque has offered call-ups to Atletico Madrid's Saul Niguez and Real Madrid's Lucas Vazquez, who are both yet to make a senior appearance for Spain, while also including Athletic Bilbao veteran Aritz Aduriz, Villarreal midfielder Bruno Soriano and Barcelona centre-back Marc Bartra.
The club went bankrupt and was refounded in 2006 as Sassari Torres 1903 and placed in Serie C2.
After the second season in C2, in 2008, the club ceased again operations due to financial troubles and was refounded with the current denomination. The club competed, for the 2008–09 season, in Promozione, the 7th level of Italian football. At the end of the season, the club gained direct promotion to Eccellenza after finishing in the first place.
"I've spoken to Torres even if he still has to do the medical," said the former Italy forward.
Brazil-born Costa was persuaded to switch allegiance to Spain ahead of the 2014 World Cup, but he has yet to enjoy any real success with La Roja after suffering injuries and disciplinary issues at club level, as well as a record of just one goal in 10 games for the country.
The club, that is the oldest football team in Sardinia, was founded in 1903 as Società di Educazione Fisica Torres and renamed in 1920 as Torres Calcio.
Forwards: Aritz Aduriz (Athletic Bilbao), Nolito (Celta Vigo), Alvaro Morata (Juventus), Lucas Vazquez (Real Madrid)
Sergio Asenjo (Villarreal), Pau Lopez (Espanyol), Hector Bellerin (Arsenal), Diego Llorente (Rayo Vallecano), Jorge Mere (Sporting Gijon), Jonny Castro (Celta Vigo), Pablo Fornals (Malaga), Marco Asensio (Espanyol), Denis Suarez (Villarreal), Inaki Williams (Athletic Bilbao), Mikel Oyarzabal (Real Sociedad)
In summer 2015 it was relegated to Serie D for sporting fraud.
The Italian club confirmed a permanent deal for Torres, following its own initial two-year loan deal, on Saturday, but have allowed him to play for Atletico after Italy winger Alessio Cerci, 27, arrived from the Spanish side.
Società di Educazione Fisica Torres 1903 (usually referred to simply as Torres) is an Italian association football club based in Sassari, Sardinia. The team currently plays in Serie D.
Milan confirmed on Twitter that Torres had landed at the city's Linate Airport, where he was greeted by 200 fans, and they posted a picture of the player holding a scarf before announcing he was at the Madonnina Clinic undergoing his physical tests.
Dermot Corrigan is a Madrid-based football writer who covers La Liga and the Spain national team for ESPN FC. Twitter @dermotmcorrigan.
"To have two finalists in the Champions League is fantastic for Spanish football," he said. "To have players in cup finals in other countries is something to be proud of [but] it affects our preparation for sure as they will not be with us until six days before the Euros start.
In 2012–13 Serie D Torres finished first in Girone G and was promoted to Lega Pro Seconda Divisione. In their first season back into professionalism, Torres could not avoid relegation after losing on playoffs to Forlì, but was successively readmitted as one of the 60 clubs of the newly unified 2014–15 Lega Pro to fill a vacancy.
The 30-year-old Spain international went through lengthy spells without scoring and has managed only 20 goals in 110 Premier League appearances since joining the club from Liverpool for a then British record £50million in January 2011.
"At last back home. Thanks to all that have made this dream come true. Forza @Atleti," he said on Twitter.
He told Milan Channel: "It’s an honour and a privilege to wear this shirt. I want to take the side back into the Champions League and perhaps stay here for many years.
In a statement, Chelsea thanked the 2010 World Cup winner for "his service during the past four seasons" and wished him "the very best for the future".
Midfielders: Bruno Soriano (Villarreal), Sergio Busquets (Barcelona), Koke (Atletico Madrid), Thiago Alcantara (Bayern Munich), Andres Iniesta (Barcelona), Isco (Real Madrid), David Silva (Manchester City), Pedro Rodriguez (Chelsea), Cesc Fabregas (Chelsea), Saul Niguez (Atletico Madrid)
"We have weighed everything up," Del Bosque said. "We do not want to call up too many people with doubts over their fitness. We have [David] Silva and Lucas Vazquez with doubts. I did not speak with anyone, as you never know where to put a limit on how many you should call."
The final 23-man squad will be announced before the deadline on May 31, and Del Bosque said he could yet change his mind before making the final call.
"I was close to joining Milan when I was at Atletico Madrid, but we’re talking about many years ago, a long time has since passed. I know the history of the club and it is illustrious. I want to make the fans happy. I couldn’t wait to arrive and start the new season. I have already spoken to the coach."
"I know some of the players like Alex, Essien, Diego Lopez. Demetrio Albertini also spoke well of Milan and he’s a great friend of mine.
Asked about the omissions, Del Bosque said: "Many people had chances, there were doubts over many of the calls, but we made the decisions we thought best.
Manchester United's Mata and Arsenal's Cazorla have been in and out of the squad through recent times due to form and injury issues and also miss out, along with Valencia centre-forward Alcacer, who was top scorer through qualifying with five goals.
Torres helped Chelsea to win the Champions League, Europa League and FA Cup while at Stamford Bridge.
The 2011–12 season was one of the most successful in the club's recent history: Torres won the Coppa Italia Sardinia and finished first in Eccellenza Sardinia and was so promoted to the 2012–13 Serie D.
Goalkeepers: Iker Casillas (Porto), David De Gea (Manchester United), Sergio Rico (Sevilla)
The women's team Torres Calcio Femminile has won six Serie A titles and eight Italian Women's Cups.
Torres is out of contract at Stamford Bridge in 2016 and the conclusion of the deal could mean the end of what has been a turbulent spell with Chelsea.
Spain coach Vicente del Bosque has left Diego Costa, Juan Mata, Fernando Torres, Paco Alcacer and Santi Cazorla out of his 25-man provisional squad for Euro 2016.
Milan coach Filippo Inzaghi is hoping he can help Torres rediscover his best from in Serie A.
"I found out what kind of person he is and it all seems very positive. It is up to me to get him back to the levels he is capable of and I will study his best position."
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Torres has enjoyed a revival at Atletico in recent months and has scored 11 La Liga goals, but it was not enough to persuade Del Bosque to recall the 32-year-old.