Pennington was brought into the action at half-time against Cardiff, following Liam Cooper’s sending off.
The Everton loanee was handed the unenviable task of having to enter the fray, with Leeds a man down and 2-0 behind in the contest.
However, Pennington coped well with Cardiff’s attacking threat - and played his part in an improved Leeds’ performance in the second-half.
Although the Whites were unable to turn the scoreline around, they did stop Cardiff running riot, with the Bluebirds ending up 3-1 winners in the contest.
And Christiansen praised Pennington’s display after the match, with the 22-year-old now likely to start for Leeds in their next outing against Sheffield Wednesday at the weekend.
“We have to replace Liam. Pennington did a very good and serious game when he came in [tonight],” Christiansen said.
Leeds’ defeat against Cardiff has seen Christiansen’s men relinquish their grip at the top of the Championship table.
The Whites have not lost back-to-back games all campaign, and will be looking to bounce back quickly against Wednesday in the Yorkshire derby on Sunday.