The Peru international, 22, is on loan at St Pauli from Schalke but could find his spell in Hamburg come to a premature end after he aimed his phlegm at Fortuna midfielder Sascha Roesler.
Zambrano was leaving the field of play after being shown a 73rd-minute red card for two bookable offences, and his mandatory one-match ban could now be extended as the DFB demanded a disciplinary hearing.
And with just eight matches remaining, Zambrano may well not feature again this season.
"I like to share things too but spitting does not belong on the football pitch," his victim Roesler said afterwards. "He really should be able to control himself.
"I’m not one to cry about stuff as I can lose my temper, but spitting is too much."
St Pauli coach Andre Schubert agreed, but team manager Christian Boenig said there would be no further internal action.
"I have spoken to him, it is very unprofessional," Schubert said.
"I’m not sure whether to be satisfied with the draw but, given the circumstances and that we had the better of the game before the sending off, the point was deserved.”
"It is an act unworthy of a St Pauli player but unfortunately it happened," Boenig added.
"We severely rebuked him but will not punish him further - it is a matter for the DFB."
Zambrano apologised profusely for the incident, saying Roesler’s provocation – the midfielder had called for the referee to show his opponent a second yellow card after a late challenge on Sascha Dum – did not warrant his act.
"It was grossly unsportsmanlike conduct that is not in my nature," he said.
"There can be many heated situations on the pitch, and in this case there were a lot of verbals building up to the incident. One thing led to another and I lost my cool – but this can be no excuse for my behaviour."
St Pauli lie one place and two points behind third-placed Fortuna.
The top two teams in the second division are automatically promoted to the Bundesliga, with the third-placed team entering a play-off with the third-bottom side in the top flight.
Eintracht Frankfurt and Greuter Fuerth are in pole position for promotion, the former leading the latter by one point with six between second and third.