The graph at the 'Real Clear Politics" poll of polls aggregator tells a story even the most partisan would find impossible to deny.
On November 4th Carson became the only candidate to pass Trump (albeit by less than 1 point and for three days) since his rise to first place on July 20th. It is a sorry story for Carson since then.
On January 2nd 2016 Dr.Carson is on 8.8% 16 points behind Trump
As things stand there is clearly no further direct threat to Trump from Carson in the polls or as an opposition candidate. The danger is that should Carson drop out in the near future it could present a very difficult hazard for Trump. If he did so before Iowa and frankly with Dr.Carson anything is possible, it would, almost certainly, give Cruz the win in this first up state.
There is no need to dwell on why Dr.Carson's campaign imploded. The basic facts of his life story coming unstuck, and his inability to deal with foreign policy questions in the debates are well known, and the graph above tells the tale. Going forwards this is the real area of concern for the Trump campaign and a possible bonanza for the Cruz team;