Simpson was released at the end of the season - departing with a glowing public reference from Johnson, who lauded the former Huddersfield forward as a person to be "in the trenches" alongside.
The 28-year-old has already received interest in his services from a number of clubs but, in the unlikely event of nothing coming to fruition, Johnson would welcome him back to Latics on reduced terms.
"He sort of made clear that if, for whatever reason, nothing works out for me over the summer then the door would always be open for me to come back," Simpson said.
"If I can't find anywhere then obviously I'd accept much lower terms. He didn't expect it to be the case and I don't either.
"The likelihood is I'm not going back so it's good of him to leave the door open."