diff --git a/onnxscript/utils/replace.py b/onnxscript/utils/replace.py
index d3af1a37a0..cab32226e7 100644
--- a/onnxscript/utils/replace.py
+++ b/onnxscript/utils/replace.py
@@ -4,14 +4,11 @@
 
 from typing import Sequence
 
-import onnx
 import onnx_ir as ir
 import onnx_ir.passes.common as common_passes
 
 
-def replace_functions(
-    model: onnx.ModelProto, functions: Sequence[onnx.FunctionProto]
-) -> onnx.ModelProto:
+def replace_functions(irmodel: ir.Model, irfunctions: Sequence[ir.Function]) -> None:
     """A utility function to replace custom operations in a model with their expansions:
     Args:
         model: An ONNX ModelProto possibly containing calls to custom operations.
@@ -20,8 +17,6 @@ def replace_functions(
     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 +27,3 @@ 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)
-    return ir.to_proto(irmodel)