He's finished, if we've got money to spend bring in a youger quality player who will be here for the next 5 years plus.
I'd only consider a loan move in the January window, until the end of the season, on the right wage structure; i.e. heavily subsidised by United. We don't know if his current form is an actual demise, or whether a move back to Everton would rejuvenate him. Also we don't know what the effect on the Goodison atmosphere would be as there's still thousands of fans who haven't, and probably never will, forgiven him for his antics after he left the club. Buying him would be a risk in my opinion.
However, it's possible that he could bring quality to the squad that we don't currently don't have in the number 10 and back up striker positions. Plus it will be difficult to find the right type of long term player we need in the January window. FFS we couldn't even do it in the summer.
I think we all know that if we don't do something in January, then our season will most likely fizzle out when we get knocked out of the cup. A loan move for Rooney may just be the short term answer and, if it works out, consider a permanent move in the summer. If it doesn't, then there's been no long term damage done.