REF: avoid mutating Series._values directly in setitem but defer to Manager method

pandas/core/internals/array_manager.py

@@ -1286,8 +1286,23 @@ def apply(self, func, **kwargs):
             new_array = getattr(self.array, func)(**kwargs)
         return type(self)([new_array], self._axes)
 
-    def setitem(self, indexer, value):
-        return self.apply_with_block("setitem", indexer=indexer, value=value)
+    def setitem(self, indexer, value, inplace: bool = False):
+        """
+        Set values with indexer.
+
+        For SingleArrayManager, this backs s[indexer] = value
+
+        Parameters
+        ----------
+        indexer, value
+        inplace : bool, default False
+            If True (for a Series), mutate the manager/values in place, not
+            returning a new Manager (and thus never changing the dtype).
+        """
+        if inplace:
+            self.arrays[0][indexer] = value
+        else:
+            return self.apply_with_block("setitem", indexer=indexer, value=value)
 
     def idelete(self, indexer) -> SingleArrayManager:
         """

pandas/core/internals/managers.py

@@ -351,8 +351,25 @@ def where(self: T, other, cond, align: bool, errors: str) -> T:
             errors=errors,
         )
 
-    def setitem(self: T, indexer, value) -> T:
-        return self.apply("setitem", indexer=indexer, value=value)
+    def setitem(self: T, indexer, value, inplace: bool = False) -> T:
+        """
+        Set values with indexer.
+
+        For SingleBlockManager, this backs s[indexer] = value
+
+        Parameters
+        ----------
+        indexer, value
+        inplace : bool, default False
+            If True (for a Series), mutate the block/values in place, not
+            returning a new Manager/Block (and thus never changing the dtype).
+        """
+        if inplace:
+            # only passed for Series (single block)
+            assert self.ndim == 1
+            self._block.values[indexer] = value
+        else:
+            return self.apply("setitem", indexer=indexer, value=value)
 
     def putmask(self, mask, new, align: bool = True):
 

pandas/core/series.py

@@ -1064,10 +1064,9 @@ def __setitem__(self, key, value) -> None:
         try:
             self._set_with_engine(key, value)
         except (KeyError, ValueError):
-            values = self._values
             if is_integer(key) and self.index.inferred_type != "integer":
                 # positional setter
-                values[key] = value
+                self._mgr.setitem(key, value, inplace=True)
             else:
                 # GH#12862 adding a new key to the Series
                 self.loc[key] = value
@@ -1099,7 +1098,7 @@ def _set_with_engine(self, key, value) -> None:
         # error: Argument 1 to "validate_numeric_casting" has incompatible type
         # "Union[dtype, ExtensionDtype]"; expected "dtype"
         validate_numeric_casting(self.dtype, value)  # type: ignore[arg-type]
-        self._values[loc] = value
+        self._mgr.setitem(loc, value, inplace=True)
 
     def _set_with(self, key, value):
         # other: fancy integer or otherwise