Chicago began Sunday six games behind division-leading Minnesota in the AL Central.
"We will play hard," he said. "I don't care what people think. Even if we're out of the pennant race, we're going to play hard. That's their job."
Chicago starts a three-game series against the visiting Twins on Tuesday.
"In this game you never know what's going to happen," Guillen said. "From Tuesday on, I don't say it's destiny, it's going to decide a lot of things — how many games we fall behind or how many games we stay in."
Guillen rested Carlos Quentin from the starting lineup Sunday, but expects him back in the lineup for the Twins series.
"We've got to be careful how we play him, because I want to keep him healthy as much as I can," Guillen said. "Carlos will play the next two days for sure. I try to mix and match in the situation."
Quentin is hitting just .236 but has 24 home runs and 82 RBIs.
Designated hitter Manny Ramirez was again in the lineup Sunday.
"I'll give a rest , but I have to play him," said Guillen. "He makes our team better."
Since joining Chicago, Ramirez had gone 7 for 24 (.292) — with all the hits singles.