The Sky Sports pundit and former Arsenal striker feels the 26-year-old will scare the United defence.
Indeed, Smith believes United's back four, which he sees as not the best back four in the world by any stretch, will be scared of dealing with the speedy Arsenal forward and those similar to him.
The Gunners face Manchester United this weekend in a clash that could be seen as a chance for both to send out a message of intent.
Smith told Sky Sports: "At the other end, Manchester United have obviously defended really well more often than not but I don't think they've got the best back four in the world by any stretch.
"They'll be scared by the likes of Theo Walcott if he's supplied with the ammunition, with the passes.
"I would imagine they would defend fairly deep and make it difficult for Arsenal to get in behind but it's going to be a fascinating clash."
Speed? Yes. Finishing? No.
As Walcott improves in the centre-forward role at Arsenal, he certainly looks a better bet right now than Olivier Giroud, while his pace will indeed frighten anyone.
But the 26-year-old has never been a natural finisher of the quality needed to play up front alone. Walcott often misses chances and, looking across the pitch at Anthony Martial, Manchester United have a forward who really can finish - even this early in his career.
United may well fear Walcott, then, but he won't strike anywhere near as much of that fear into Louis van Gaal's defence as a real world-class centre-forward would.