Being critical you can apportion blame to Sakho for his inadequate initial header and subsequently poor positioning however, this goal was largely down to an unlikely sequence of fortunate events for the Swansea midfielder. (anfieldindex.com)
There is also a push in the back of the defender prior to the initial header. (justarsenal.com)
For the first goal Monreal was a bit weak in competing for the initial header which Cech saved and Mustafi failed to follow the run of Chalov, who scored from the rebound. (justarsenal.com)