Despite a good season individually - Rooney has scored 17 goals and 10 assists in the league - United have struggled in their first season without the leadership of Alex Ferguson.
United are planning an overhaul of the squad this summer, hoping to add to existing quality players like Rooney, Robin van Persie and Mata.
And England international Rooney has vowed to show the United faithful a much improved side next season.
Speaking on his official Facebook account, Rooney said: "It's been a tough season but we will be back much stronger next year!"
After appointing David Moyes to succeed fellow Scot Alex Ferguson, the 20 time English champions have struggled, and failed to qualify for even the Europa League this season.
However, Rooney also wanted to thank the United fans, adding "Just want to say thank you to all the fans for their loyal support this season."
28-year-old Rooney will now be looking to finish his season on a high as he travels to Brazil with England.
For Manchester United, however, the work begins to try and rebuild a squad capable of wrestling the title away from newly-crowned champions, and rivals, Manchester City.
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