diff --git a/onnxscript/utils/replace.py b/onnxscript/utils/replace.py
index d3af1a37a0..d46493155d 100644
--- a/onnxscript/utils/replace.py
+++ b/onnxscript/utils/replace.py
@@ -9,19 +9,17 @@
 import onnx_ir.passes.common as common_passes
 
 
-def replace_functions(
-    model: onnx.ModelProto, functions: Sequence[onnx.FunctionProto]
-) -> onnx.ModelProto:
+def replace_functions_inplace(irmodel: ir.Model, irfunctions: Sequence[ir.Function]) -> None:
     """A utility function to replace custom operations in a model with their expansions:
+
+    The model is updated in-place.
+
     Args:
-        model: An ONNX ModelProto possibly containing calls to custom operations.
-        functions: A sequence of FunctionProto defining the expansions for the custom operations.
+        irmodel: An ONNX model possibly containing calls to custom operations.
+        irfunctions: A sequence of functions defining the expansions for the custom operations.
+
 
-    Returns:
-        An updated ModelProto with custom operations replaced by their expansions.
     """
-    irmodel = ir.from_proto(model)
-    irfunctions = [ir.from_proto(func) for func in functions]
     model_functions = irmodel.functions
     if len(model_functions) != 0:
         # Since we use inlining, check that there are no model-local functions.
@@ -32,4 +30,20 @@ def replace_functions(
     # TODO (rama): Ideally, we should provide users more control over renaming strategy for inlined values.
     common_passes.InlinePass()(irmodel)
     common_passes.RemoveUnusedOpsetsPass()(irmodel)
+
+
+def replace_functions(
+    model: onnx.ModelProto, functions: Sequence[onnx.FunctionProto]
+) -> onnx.ModelProto:
+    """A utility function to replace custom operations in a model with their expansions:
+    Args:
+        model: An ONNX ModelProto possibly containing calls to custom operations.
+        functions: A sequence of FunctionProto defining the expansions for the custom operations.
+
+    Returns:
+        An updated ModelProto with custom operations replaced by their expansions.
+    """
+    irmodel = ir.from_proto(model)
+    irfunctions = [ir.from_proto(func) for func in functions]
+    replace_functions_inplace(irmodel, irfunctions)
     return ir.to_proto(irmodel)