De Gea's move to Real was wrecked in bizarre circumstances on Monday when the paperwork was not delivered until after the midnight deadline.
Real and United blamed each other for the bureaucratic snafu, which means De Gea will remain in Manchester, while Real's Costa Rica keeper Keylor Navas, who was meant to join United as part of the deal, stays in Madrid.
De Gea, whose United contract expires at the end of the season, could find himself in limbo, at least until the end of the year when the market opens again.
While the transfer saga rumbled on, United coach Louis van Gaal left the 24-year-old former Atletico Madrid keeper out of the squad, with Argentina's Sergio Romero playing instead.
"I don't want to say anything drastic about what we will do," Del Bosque, who has hinted De Gea will not be selected if he is not playing regularly, told Spanish radio.
"We have to wait and see what happens and the important thing is that David... looks to the future and is a good professional," he added.
"We have to hope that David can play for (United) and is active so we can count on him in future (Spain) squads and he can show what a good keeper he is.
"It's a difficult situation for him but he has to be happy to be in the national team and at (United) and in the future he will be able to decide where he would like to go."
De Gea has yet to comment publicly on the failed deal and did not speak to reporters when he arrived at Spain's training camp outside Madrid on Tuesday.
With captain Iker Casillas also in the squad, De Gea is likely to be on the bench for Spain's Euro 2016 qualifiers at home to Group C leaders Slovakia on Saturday and away to Macedonia three days later.
(Reporting by Iain Rogers, editing by Sudipto Ganguly)
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