diff --git a/pandas/_libs/reduction.pyx b/pandas/_libs/reduction.pyx
index 4d0bd4744be5d..5649d1378cda3 100644
--- a/pandas/_libs/reduction.pyx
+++ b/pandas/_libs/reduction.pyx
@@ -66,9 +66,7 @@ cdef class _BaseGrouper:
             object.__setattr__(cached_ityp, '_index_data', islider.buf)
             cached_ityp._engine.clear_mapping()
             cached_ityp._cache.clear()  # e.g. inferred_freq must go
-            object.__setattr__(cached_typ._mgr._block, 'values', vslider.buf)
-            object.__setattr__(cached_typ._mgr._block, 'mgr_locs',
-                               slice(len(vslider.buf)))
+            cached_typ._mgr.set_values(vslider.buf)
             object.__setattr__(cached_typ, '_index', cached_ityp)
             object.__setattr__(cached_typ, 'name', self.name)
 
diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index 2ad7471d6f086..09559e571d5ee 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -1654,6 +1654,7 @@ def set_values(self, values: ArrayLike):
         valid for the current Block/SingleBlockManager (length, dtype, etc).
         """
         self.blocks[0].values = values
+        self.blocks[0]._mgr_locs = libinternals.BlockPlacement(slice(len(values)))
 
 
 # --------------------------------------------------------------------