The pair initially argued after an incident on the field, with Lewandowski furious with the Frenchman.
It's understood the issue arose after Lewandowski let rip at Coman during a training exercise, criticising the France international for not taking the session seriously, reports the Mirror.
That confrontation soon became physical however, with Coman striking the Polish forward, who responded by landing a strike of his own on the 22-year-old.
The pair were initially broken apart by both Jerome Boating and Niklas Sule.
However, tensions continued to run high and moments after being split apart, the duo had to be separated again, with Bild reporting that a number of other teammates had to subsequently intervene.