Kersken went down without any involvement in the 69th minute on Sunday and received treatment, with Arminia coach Mitch Kniat taking the opportunity to talk to his players.
Viktoria Cologne coach Olaf Janssen, who was sent off for protesting Kersken's injury, said after the game the keeper was not injured and did it so that Kniat could confer with his players, calling the move 'unsportsmanlike'.
Kniat, whose team face VfB Stuttgart in the German Cup final on May 24th, told a press conference the keeper's actions fell in a "grey zone" of regulations and he said he was just taking advantage of that.
"I have learned two words from referees all these years that I am a coach. Grey zones... so we can use the grey zones for us as well," Kniat said.
The DFB, however, has now contacted the coach, demanding clarification on the matter, a DFB spokesperson told German media.
According to the rules, a keeper is treated for an injury on the pitch, in contrast to outfield players who must be treated on the sidelines so that play can continue.