Manipulate
needs to see the parameters. This is mentioned in the 'Potential issues' part of its doc page. However, I think (but currently can't test this) that including a dummy statement using the parameters may work:
foo = Exp[2 x+y]x + y]
Manipulate[
{x, y};Plot[foo, {x, 0, 10}], {y, 0, 10},
LocalizeVariables ->False]> False]