From
this interview:
"Hoy hemos anunciado que tendriamos a estos dos personajes jugables en Sonic Generations, pero también aparecerán muchos personajes del universo Sonic. Hablo de amigos de sonic: Tails, Knuckles, Shadow, etc. que formarán parte de la historia o serán un elemento importante del juego"
My translation:
"Today we have announced that we' have these 2 playable characters in Sonic Generations, but also, many characters from the Sonic universe will make an appearance. I'm speaking of Sonic's [shitty] friends: Tails, Knuckles, Shadow, etc. who will take part in the story or will be an important element of the game"