The Englishman - who recently made his loan move from Blackpool permanent - played through the problem in late 2017 but hasn't featured since aggravating the problem following his last appearance against Hamilton on December 30.
"Peter will be someone we monitor," Robinson said.
"We're not going to rush him back. He's an injury we feel needs a lot of rest.
"My aim now is to make sure he's right for next season.
"If he gets a game before the end of the season it's a bonus but first and foremost make sure he's right to go next season."
Hartley (29) penned a new contract until the summer of 2020 in January, just reward for a string of excellent appearances in the first few months of the season.
Although his absence is a definite blow for the Steelmen, new signing Tom Aldred has been similarly impressive since coming into the Motherwell backline and has helped the Fir Park outfit win six games during an excellent start to 2018.