On Thursday, Mateo threw five shutout innings as an emergency reliever for Carlos Zambrano, who left that game in the second inning with a sore back. Mateo struck out six that day while throwing 57 pitches. Zambrano went on the disabled list on Friday.
The Cubs will need to make a roster move on Tuesday regardless of Mateo's condition, when right-hander Ramon Ortiz is added to the roster to start the second game against the Nationals.
Bruce Levine covers the Cubs for ESPNChicago.com and ESPN 1000.