Defensive midfielder Osman has been a key player for the Cobblers since joining from Gretna in 2008 but was dropped for the midweek draw at Burton as Johnson gave Nathaniel Wedderburn a start instead.
Johnson said: "Abdul Osman paid the price a little bit for his performance at Accrington last Saturday. I like him as a player and as a person but he's just got to do a little bit more.
"I've seen him play in seven games now and he needs to produce more effective passing and create some goals, get some assists and some goals.
"I was pleased with Nat, he earned his place and had a good game at Burton but he blew up after an hour and he ran out of legs.
"I've got him running a mile a day because I want him to change his body shape so he was likely to be a bit tired. I thought he did okay the other night."
Source: PA
Source: PA