Kalas impressed in victory at Liverpool.
Keown told the Daily Mail: "Defending the Kop end at Anfield is one of the hardest jobs in football for an away side. To throw a young defender into that situation was a bold move but Tomas Kalas responded with a superb display.
"He was committed and able to seek out danger quickly. It was a huge risk but Kalas repaid Mourinho's faith."