The pair play undercover cop Vincent Downs and Internal Affairs investigator Jennifer Bryant in the upcoming crime thriller and as they filmed one particularly action-packed fight scene Foxx was left with a gap in his Hollywood smile and a bleeding gum after Monaghan knocked out one of his gnashers.
Speaking to website Collider, she said: "I was a little quick on the draw. He told me, 'Keep going.' I had split my knuckle. I was like, 'Alright. It hurts!' "
The 40-year-old actress was very worried about how the dental dilemma would affect shooting but when she arrived on set the next day she found Foxx, 49, with a perfect set of pearly whites once again.
She added: "It was funny the next day because we didn't bring it up. I felt very sheepish. I was trying to cop a look and I was like, 'Oh my God, it's OK.' Twelve hours later, he looked brand new."
In the joint interview, Foxx revealed he was able to get his dental disaster solved so quickly because he one of his best friends is a dentist who lives in Atlanta, where 'Sleepless' is set was filmed, and was able to get him fixed up immediately so filming wouldn't be delayed.
He said: "My best friend lives in Atlanta. He's a dentist. My man came and hooked me up and took me to the office and put the porcelain back on."
'Sleepless' - directed by Baran bo Odar - will be released on January 13 in the US and then across the rest of the world at a later date.