Johnson has outlined his wish to follow suit to have a chance at winning the Super Bowl before the end of his career, which is slowly approaching.
Foster told KPRC-TV: "I understand where he's coming from. I really do. This is a business and people forget this is a business and he's at a point in his career where he doesn't want money now. He wants to go win.
"I would love to have him here, but ultimately that's one of my friends outside of the field. I want him to do what's best for him.
"If he doesn't feel like playing anymore and wants to hang them up, I'm going to support him 100 percent. If he wants to come back and help us win, I'm all for that as well."
The 33-year-old recorded 109 receptions for 1,407 yards and five touchdowns last season.