Nowak already knew at the start of the week that he would be without Williams due to a concussion, and Mondragon has been out for weeks with a broken finger.
Then on Wednesday, Adu sprained an ankle at practice, forcing him to stay home. Mwanga was also ruled out of the trip with a flare-up of a recurring hip injury.
The Union (9-7-13, 40 points) spent much of the game under pressure from Kansas City's fast and skilled forwards. But central defenders Danny Califf and Carlos Valdes marshaled the back line, and Zac MacMath made a number of impressive saves.
Only once in the first half did MacMath really slip up, but Kansas City (10-9-11, 41 points) failed to capitalize. In the 40th minute, MacMath fumbled a cross out of bounds for a corner kick. The ensuing header from Kei Kamara was just a few inches too high and hit the crossbar.
MacMath was not at fault when Kansas City finally broke through in the 56th minute, and it became just how clear the Union missed Williams. Kamara received a pass with no one near him on the left side of the 18-yard box - usually Williams' territory to defend - and laid a perfect cross for Omar Bravo to tap in.
The Union struck back just seven minutes later. Roger Torres played a low cross through the box that nearly went out of bounds, but Michael Farfan kept it in play by inches. He laid a pass back for Le Toux, who slotted a first-time shot past Sporting goalkeeper Jimmy Nielsen's left side.
That silenced almost the entire crowd at Kansas City's spectacular new home, which opened in June at a cost of $200 million. The only noise in the stadium came from the approximately 50 members of the Union's Sons of Ben supporters club who traveled west for the occasion.
Sporting surged forward repeatedly after that, but MacMath made a number of impressive saves to preserve the tie - and keep the Union's hopes alive of gaining an automatic playoff spot as one of the Eastern Conference's top three teams.