The Watford goalkeeper added that the Serbian was anything but scary off the training pitch.
"Just talking about Nemanja, off the pitch he was fairly quiet wasn't he, but on the pitch, in training, that guy was a scary bloke, honestly," Foster told the Squeaky Bum Time podcast.
"Horrible, honestly he did not care. If we were playing a five-a-side or six-a-side or something, if you were on his team, I used to think "please don't be on Nemanja's team, please don't".
"Because if we didn't win, if I let a goal in that he thinks I should have done better with, he was letting you know and he would dig you out and he was after everybody.
"Honestly he was horrible in training but off the pitch completely different."