Mata, who has been struggling for consistent form since his move from Chelsea in January, was omitted from Del Bosque's squad for the friendly with Italy earlier this month.
The former Valencia schemer admits the alarm bells are ringing and fears are growing that he won't be on the plane to Brazil with La Roja.
Speaking with Spanish radio Onda Cero, Mata was quoted by Marca: "It's hard.
"It has always been hard, ever since my first time. I expect it will have been very difficult for Del Bosque to pick the 23. Luckily I have been in all the big competitions since I made my debut and my hopes are still intact."
United have drawn Pep Guardiola's Bayern Munich in the Champions League quarterfinals.
Bayern are sure to feature Mata's Spain rivals Javi Martinez and Thiago Alcantara in their midfield - a frustrating irony for Mata as he will be Cup-tied from both legs after appearing for Chelsea in the competition earlier this season.
Despite Bayern being overwhelming favourites, Mata has the experience of pulling off the improbable in Europe and is urging his United teammates to keep believing.
"The whole world knows how tough it will be, but I recall my first year at Chelsea, when we won the Champions League against the odds. Let's wait and see."
The midfielder is expected to start his ninth Premier League game for United tonight against Manchester City.
At £37 million, Mata carries the burden of being United's record transfer, but the diminutive playmaker says he is loving the experience and is 'amazed' by the support the club enjoys around the world.
"Manchester United is always going to have great players, because of its history and the reputation it has around the world. When you are on the outside you realise what a big club it is, but when you are on the inside is amazing to see the level of support there is around the world."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball