The Spaniard's tough loan spell was exacerbated when he was dragged off in the 2-1 victory.
But Tuchel insists the decision had nothing to do with Saul's performance.
"He was on a yellow card," Tuchel said.
"We had two options, [the other was to] take Marcos off and put Saul as a wing-back. I had this in my mind before the game if something happened to Marcos, but it wasn't the right match to try new positions.
"It was him [who went off] to change the shape a little bit and have Thiago on the pitch with the quality of organising, and Trevoh in the six who is more physical.
"Unfortunately Trevoh injured himself so we ended up with Ruben in the single six. It was a tactical decision and we go on from here."