As the Nets take the floor tonight at United Center for Game 6 of their first-round series against the Bulls — one they hope to push to a seventh game back in Brooklyn on Saturday night — they’ll do so with one of their key players severely limited.

If the Nets were in the middle of the regular season, as opposed to one loss away from their season being over, Joe Johnson wouldn’t be taking his customary place in the starting lineup tonight alongside Deron Williams in the Nets’ backcourt because of a lingering case of plantar fasciitis in his left foot.

“Probably not,” Johnson said after yesterday’s practice. “Probably not.

“When it first came about in February, we tried to rest it as much as possible, but obviously it wasn’t enough time. I’m just giving them everything I’ve got at this point.”

Johnson initially suffered a more mild form of the injury, one the Nets described as a sore left heel, in mid-February. He sat out for four games a short time later, only to sit for several more in late March and early April thanks to a combination of the heel and a right quad contusion he suffered after running into Blake Griffin in a game against the Clippers on March 23.

By the end of the regular season, however, Johnson was feeling much better, and was happy with the way his foot had responded heading into the playoffs. That changed, though, after landing awkwardly while making a 3-pointer in the first quarter of the Nets’ 90-82 loss to the Bulls in Game 2.

“There’s times where I do have to push off a lot. … There’s times when I have to push off my foot and make plays and do what I have to do to help this team,” Johnson said. “So, as much as it bothers me, I just play through it.”