Interest from Anfield has been well documented up until now but this is the first mention of the Manchester clubs getting involved. It is further evidence that the situation around Walcott's future is starting to escalate ahead of a summer of uncertainty.
Walcott has just 12 months left on his Arsenal contract and took to Twitter to claim that he had yet to receive an offer from the club. That contradicted a claim from Arsene Wenger, who said that preliminary discussions had indeed started.
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The England international currently earns £100,000-a-week at the Emirates and came very close to leaving the club before agreeing that deal. Walcott had just five months left on his contract when he agreed the three-and-a-half year deal.
Since then Walcott has only gone backwards. A serious knee injury kept him on the sidelines for a year and he return to full fitness to find Alex Oxlade-Chamberlain and Alexis Sanchez ahead of him in the pecking order.
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Now fit to return to action, Walcott has struggled for game time and has only started once this month. Against Everton, QPR, Manchester United and Newcastle United, he watched on from the bench, which would have been a big concern for the forward.
His knee injury has does not seem to have affected his explosive pace, but there are big questions over his long-term role for the Gunners. He will not want to be a super-sub and Manchester United are ready to offer him an alternative.
Manchester United have a history of stealing important Arsenal players after convincing Robin van Persie to swap to Emirates for Old Trafford in 2012. Van Persie has been a success for the Red Devils and Louis van Gaal could see Walcott as a formidable partner for him.
They will have to beat Manchester City and Liverpool to his signature, who are both interested as well. Walcott would go for big money this summer if all three clubs come in. His status as an established homegrown star leaves him as a valuable asset for anyone.
Regardless of his form, losing Walcott would be a massive loss for Wenger and Arsenal. The Gunners have worked hard to lose their tag as a selling club, but the departure of Walcott would undo all the hard work that has been done in the transfer market since the departure of Van Persie.
Where will Theo Walcott be next season? Give us your views in the comment box below!