I know that expressionism distorts forms and colors but I would never compare Grunewald’s paintings with Expressionists as I looked at his paintings.
Many artists and art historians have made this connection.
El Greco is regarded as a precursor of Expressionism.
Yes... he is. And you'll note that I included the Mannerists and El Greco would fall under that category. On the other hand, El Greco probably didn't have a huge impact on artists outside of Spain & Picasso for the simple reason that the vast majority of his paintings are housed in Spain and somewhat inaccessible to artists in France or Germany and Spain wasn't much of a destination for artists from outside in the 19th or 20th centuries. Oddly enough, Manet did make a tour of Spain and was quite impressed with his work... as well as with that of Velazquez (which is more obvious).
I was able to see a large portion of El Greco's oeuvre as part of a retrospective shown at the Met some years back. The best paintings do indeed have a sense of a tormented spirituality. The worst paintings, however, have something of black velvet paintings about them. I loved his flickering brushwork... but was disappointed in his use of color. He was an even worst colorist than his teacher, Tintoretto.
While we're on Spain... we might point out that Goya was another precursor of Expressionism: