Spanish trickster Saiz was back available after missing United’s last two games with a calf problem but the 26-year-old only returned to the bench as Pablo Hernandez kept his place in the no 10 role with Pawel Cibicki and Gjanni Alioski keeping their shirts out wide.
With Leeds desperately struggling to create clearcut chances, Saiz was finally brought on for Cibicki ten minutes into the second half, after which United’s attacking prowess improved.
Saiz set up United’s best chance of the game for Pablo Hernandez in the 77th minute but Hernandez’s attempt was put on to the crossbar by City ‘keeper David Stockdale and Birmingham’s Jacques Maghoma then scored the only goal of the game six minutes later.
Asked if he had considered starting Saiz, Christiansen said: “No. I thought about it but I believed that at this time, with so many games, it was a game to let it mature a little bit so that in the second half he will be fresh and the possibility to make the difference which he did.
“It was a big change but he came in in a situation where Birmingham were not so fresh and he would have space.
“From this came the best opportunities.”
Reflecting on United’s first defeat in seven games, Christiansen said: “It was from my point of view a horrible game with direct play, challenges and not really big opportunities.
“There was one very good save from each goalkeeper but that was not our game today.
“It was a difficult game.
“We knew what the opponent was going to do with (Sam) Gallagher up front.
“He was the target man always trying to make the second ball.
“We didn’t stay close enough to pick up these balls and when we did we lost it immediately with difficult and dangerous passes.
“That gave them the possibility to go into transition.”