diff --git a/mlir/lib/Tools/PDLL/CodeGen/MLIRGen.cpp b/mlir/lib/Tools/PDLL/CodeGen/MLIRGen.cpp
--- a/mlir/lib/Tools/PDLL/CodeGen/MLIRGen.cpp
+++ b/mlir/lib/Tools/PDLL/CodeGen/MLIRGen.cpp
@@ -444,11 +444,19 @@
 
     assert(opType.getName() && "expected valid operation name");
     const ods::Operation *odsOp = odsContext.lookupOperation(*opType.getName());
-    assert(odsOp && "expected valid ODS operation information");
+
+    unsigned resultIndex;
+    if (!odsOp) {
+      assert(llvm::isDigit(name[0]) && "unregistered op only allows numeric indexing");
+      name.getAsInteger(/*Radix=*/10, resultIndex);
+      IntegerAttr index = builder.getI32IntegerAttr(resultIndex);
+      return builder.create<pdl::ResultOp>(loc, genType(expr->getType()),
+                                            parentExprs[0], index);
+    }
 
     // Find the result with the member name or by index.
     ArrayRef<ods::OperandOrResult> results = odsOp->getResults();
-    unsigned resultIndex = results.size();
+    resultIndex = results.size();
     if (llvm::isDigit(name[0])) {
       name.getAsInteger(/*Radix=*/10, resultIndex);
     } else {
diff --git a/mlir/lib/Tools/PDLL/Parser/Parser.cpp b/mlir/lib/Tools/PDLL/Parser/Parser.cpp
--- a/mlir/lib/Tools/PDLL/Parser/Parser.cpp
+++ b/mlir/lib/Tools/PDLL/Parser/Parser.cpp
@@ -2683,8 +2683,11 @@
       });
       if (it != results.end())
         return it->isVariadic() ? valueRangeTy : valueTy;
+    } else if (llvm::isDigit(name[0])) {
+      // For unregistered op, we allow unchecked numeric indexing and it'll be
+      // supposed to be a single value.
+      return valueTy;
     }
-
   } else if (auto tupleType = parentType.dyn_cast<ast::TupleType>()) {
     // Handle indexed results.
     unsigned index = 0;
diff --git a/mlir/test/mlir-pdll/CodeGen/MLIR/expr.pdll b/mlir/test/mlir-pdll/CodeGen/MLIR/expr.pdll
--- a/mlir/test/mlir-pdll/CodeGen/MLIR/expr.pdll
+++ b/mlir/test/mlir-pdll/CodeGen/MLIR/expr.pdll
@@ -55,6 +55,19 @@
 
 // -----
 
+// Handle result indexing on unregistered op.
+// CHECK: pdl.pattern @UnregisteredOpResultIndexing
+// CHECK: %[[BAR_OP:.*]] = operation "unregistered.bar"
+// CHECK: %[[BAR_RES:.*]] = result 0 of %[[BAR_OP]]
+// CHECK: operation "unregistered.foo"(%[[BAR_RES]] : !pdl.value)
+Pattern UnregisteredOpResultIndexing {
+  let bar : Op<unregistered.bar>;
+  let op = op<unregistered.foo>(bar.0);
+  erase op;
+}
+
+// -----
+
 // Handle implicit "named" operation results access.
 
 #include "include/ops.td"
diff --git a/mlir/test/mlir-pdll/Parser/expr.pdll b/mlir/test/mlir-pdll/Parser/expr.pdll
--- a/mlir/test/mlir-pdll/Parser/expr.pdll
+++ b/mlir/test/mlir-pdll/Parser/expr.pdll
@@ -90,6 +90,26 @@
 
 // -----
 
+// CHECK: Module
+// CHECK: `-VariableDecl {{.*}} Name<op> Type<Op<unregistered.foo>>
+// CHECK:   `-OperationExpr {{.*}} Type<Op<unregistered.foo>>
+// CHECK:     `-OpNameDecl {{.*}} Name<unregistered.foo>
+// CHECK:     `Operands`
+// CHECK:       `-MemberAccessExpr {{.*}} Member<0> Type<Value>
+// CHECK:         `-OperationExpr {{.*}} Type<Op<unregistered.bar>>
+// CHECK:           `-OpNameDecl {{.*}} Name<unregistered.bar>
+// CHECK:           `Operands`
+// CHECK:             `-DeclRefExpr {{.*}} Type<ValueRange>
+// CHECK:               `-VariableDecl {{.*}} Name<_> Type<ValueRange>
+// CHECK:                 `Constraints`
+// CHECK:                   `-ValueRangeConstraintDecl
+Pattern {
+  let op = op<unregistered.foo>(op<unregistered.bar>.0);
+  erase op;
+}
+
+// -----
+
 //===----------------------------------------------------------------------===//
 // OperationExpr
 //===----------------------------------------------------------------------===//