diff --git a/doc/source/whatsnew/v2.0.0.rst b/doc/source/whatsnew/v2.0.0.rst
index 46428dcf462ea..121e07957b72a 100644
--- a/doc/source/whatsnew/v2.0.0.rst
+++ b/doc/source/whatsnew/v2.0.0.rst
@@ -1062,8 +1062,8 @@ Indexing
 - Bug in :meth:`Series.rename` with :class:`MultiIndex` losing extension array dtypes (:issue:`21055`)
 - Bug in :meth:`DataFrame.isetitem` coercing extension array dtypes in :class:`DataFrame` to object (:issue:`49922`)
 - Bug in :class:`BusinessHour` would cause creation of :class:`DatetimeIndex` to fail when no opening hour was included in the index (:issue:`49835`)
+- Bug in :meth:`Series.loc` casting :class:`Series` to ``np.dnarray`` when assigning :class:`Series` at predefined index of ``object`` dtype :class:`Series` (:issue:`48933`)
 -
-
 Missing
 ^^^^^^^
 - Bug in :meth:`Index.equals` raising ``TypeError`` when :class:`Index` consists of tuples that contain ``NA`` (:issue:`48446`)
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index afbf2735865dc..7ef93833e91d3 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -939,8 +939,11 @@ def setitem(self, indexer, value) -> Block:
 
         # length checking
         check_setitem_lengths(indexer, value, values)
-
-        value = extract_array(value, extract_numpy=True)
+        if self.dtype != _dtype_obj:
+            # GH47425
+            # np_can_hold_element assumes that extract_array has been called on
+            # non-object dtype value
+            value = extract_array(value, extract_numpy=True)
         try:
             casted = np_can_hold_element(values.dtype, value)
         except LossySetitemError:
diff --git a/pandas/tests/indexing/test_indexing.py b/pandas/tests/indexing/test_indexing.py
index 230605d2f3235..d0c2a983e7fa4 100644
--- a/pandas/tests/indexing/test_indexing.py
+++ b/pandas/tests/indexing/test_indexing.py
@@ -1106,3 +1106,19 @@ def test_scalar_setitem_series_with_nested_value_length1(value, indexer_sli):
         assert (ser.loc[0] == value).all()
     else:
         assert ser.loc[0] == value
+
+
+def test_object_dtype_series_set_series_element():
+    # GH48933
+    s1 = Series(dtype="O", index=["a", "b"])
+
+    s1["a"] = Series()
+    s1.loc["b"] = Series()
+
+    tm.assert_series_equal(s1.loc["a"], Series())
+    tm.assert_series_equal(s1.loc["b"], Series())
+
+    s2 = Series(dtype="O", index=["a", "b"])
+
+    s2.iloc[1] = Series()
+    tm.assert_series_equal(s2.iloc[1], Series())