De Gea, who has yet to win a full cap, is likely to miss out on the main event and go to the London Olympics instead with Del Bosque still to name almost a dozen players from Chelsea, Barcelona and Athletic Bilbao who have matches left to play this season.
Chelsea's Juan Mata and Fernando Torres will find out whether they are selected after the Champions League final on May 19, Barca and Bilbao players after the Copa del Rey final on May 25, which is sure to include goalkeeper Victor Valdes.
The world and defending European champions will name their final 23-man list on May 27.
Isco and Ignacio Monreal of Malaga, Benat of Real Betis, Javi Garcia of Benfica and three players from Europa League winners Atletico Madrid were among the unfamiliar faces in the squad.
"Obviously it will be a little painful for some, but we preferred this system of discards," Del Bosque said at Spain's training ground outside Madrid.
"We aren't only thinking of the Euro 2012, but of the future and this call up is like a reward. In September we start qualifying for the World Cup in Brazil and we will need alternatives.
"Obviously we are worried there could be an emotional hangover for some players," he added, alluding to the lack of preparation time after the finals for some stalwarts of the team.
"We aren't doing a pre-season, rather extra-time on the season we have just finished. Our obligation is to adapt to the circumstances."
With talismanic centre-back Carles Puyol ruled out with a knee injury and striker David Villa still not back in action after breaking his leg last December, the newcomers in defence and up front have the best options to secure a finals place.
Del Bosque gave little away on Villa's recovery, who could return to play for Barca in the Cup final.
"He is a special case. We will decide on the 27th," he said. "He is recovering and he could make Poland and Ukraine. We value his efforts and his desire to be here."
Spain will travel to Austria as part of their preparations and will play friendlies in Switzerland against Serbia in St. Gallen on May 26, and South Korea in Berne four days later. They return for a final friendly against China in Seville on June 3.
Spain won all eight of their qualifiers for Euro 2012 and are in Group C for the finals along with Italy, against who they play their opening game on June 10, Republic of Ireland and Croatia.
Goalkeepers: Iker Casillas (Real Madrid), David De Gea (Manchester United), Pepe Reina (Liverpool)
Defenders: Alvaro Arbeloa (Real Madrid), Sergio Ramos (Real Madrid), Jordi Alba (Valencia), Raul Albiol (Real Madrid), Alvaro Dominguez (Atletico Madrid), Ignacio Monreal (Malaga) Juanfran (Atletico Madrid)
Midfielders: Isco (Malaga), Xabi Alonso (Real Madrid), Santi Cazorla (Malaga), Benat (Real Betis), Javier Garcia (Benfica), Bruno Soriano (Villarreal)
Forwards: Adrian Lopez (Atletico Madrid), David Silva (Manchester City), Roberto Soldado (Valencia), Jesus Navas (Sevilla), Alvaro Negredo (Sevilla)