Rooney has scored six non-penalty goals and assisted four this season, for a rate of a little less than a goal or assist per 90 minutes. These are solid numbers, but the league’s top strikers Sergio Aguero and Romelu Lukaku run in the range of 0.8 or 0.9 goals plus assists per 90 minutes. Rooney’s rate of goal contribution places him 24th in the league.
Further, these numbers may flatter Rooney slightly as his teammates have done well to finish four goals off his assist passes. By expected goals, a measure of the quality of the chances Rooney has assisted or gotten on the end of, the Manchester United striker is 36th in the league with 0.39 xG+xA/90, behind lower-table strikers such as Aleksandar Mitrovic and Jermain Defoe.
Fundamentally, this just is not the Rooney who helped lead Manchester United to multiple Premier League titles. That player appears to have gone into decline along with his team. Rooney’s chance contribution drops off clearly in the middle of the 2012-13 season. The dominant attacking force of the previous two seasons never quite returned since then. It is possible this has to do with normal decline, or with Rooney’s struggles in the systems of new managers David Moyes and Louis van Gaal (Moyes took over for Sir Alex Ferguson after the 2012-13 season). But it is notable that Rooney’s decline corresponds almost exactly to his missing nearly a month of time to a knee injury from December 2012 to January 2013. This can be no more than hypothesis, but it fits the data extremely well.
The following table displays a rolling average of Rooney’s xG and xA per 90 minutes played, and the extent of his decline in the last three seasons is clear.
The recent hot streak that had Rooney seeming rejuvenated, with six goals since the new year, does appear on this chart. But it takes him merely up to his better post-Ferguson level, not to the elite level at which he used to produce. The most likely scenario is that this has merely been a hot streak by a still-diminished player rather than the beginning of a radical return.
So, Manchester United probably will not miss Rooney much. In Anthony Martial, the club has an attacker averaging better numbers than Rooney (0.47 xG+xA/90, 0.5 NPG+A/90) while playing from the wing. If Martial can raise his production playing through the middle, as is common, that could already make up for a good chunk of the loss.
The problem for United in the league is that the club is already six points back of Manchester City in fourth place. Just getting back into the race will require a serious hot streak, and while Rooney cannot carry a team there himself, he offered one more weapon, one more useful player who might contribute to that hot streak. The injury to Rooney, then, is a marginal loss rather than a huge change, but United is not in position to take many more marginal losses before its shot at Champions League play next year is entirely gone.