The 32-year-old played no part in Saturday's 1-0 friendly win at Rotherham after picking up the problem on the previous day's training.
Robins told the club's official website: "We lost Keith Southern to a bit of a calf strain on Friday, which was a big disappointment.
"I think he will miss between a week and 10 days, but at this stage you want him to be able to work and be sharp.
"He's put the work in during pre-season and got to a stage where he looked fit again, only to half a problem with his calf."
Robins was also hopeful that injured duo Jon Stead and Oscar Gobern are on the road to recovery.
"Hopefully Jon Stead will be back within 10 days, but I do get Oscar Gobern back on Monday," Robins added.
"It was a blow losing him, but he's been working through pre-season and will have a base of fitness - it's just about contact for him now."
The Terriers won their first game of pre-season against Robins' former club thanks to Martin Paterson's late winner.