A 3-2 home defeat to Roma, decided by a Vasilis Torosidis goal before Matteo Contini was sent off, saw the Nerazzurri slip to within six points of the bottom three having won just once in 10 games.
Only Genoa's failure to beat bottom side Palermo spared Atalanta from finishing the weekend in 17th place and Colantuono knows the lucky escape was a one-off.
He told La Repubblica: "We are a little bit calmer because the gap hasn't changed but now we must absolutely get back to picking up points in any which way we can.
"We matched Roma but we made a dreadful mistake on the third goal which cost us valuable points and could prove to be decisive."
Midfielder Riccardo Cazzola joins defender Contini in the stands as he sits out a one-match ban, with Colantuono running the rule over Diego Capelli, Luca Cigarini, Gian Bellini and Facundo Parra, but the season is definitely over for striker Guido Marilungo.
Contini misses out after receiving his marching orders in Rome and the 32-year-old faces an agonising 90 minutes as a spectator at the Stadio Artemio Franchi.
"We were hoping to at least get a draw against Roma, but instead it went really badly," he told the Corriere dello Sport.
"Now we must absolutely not lose to Siena. Without the points deduction they would be having a totally different season and we must go there to give them a game, knowing that we just cannot afford to lose."
A 3-0 defeat at league leaders Juventus was not entirely unexpected for Giuseppe Iachini's Siena, who have reached 18th place, but the former Sampdoria boss hopes the heavy scoreline does not have a demoralising effect on his players.
"They were more cynical than us and they took advantage of that fact whenever they could," he said. "The end result was a little unfair, even if Juve are a great team and very difficult to deal with.
"Nothing changes for us and we must continue on the path towards securing our survival."
Genoa and safety are just five points away but injured midfielder Simone Vergassola will play no part as Siena bid for a first home win over Atalanta since January 2009.