The 26-year-old Robinson will be allowed to play in seven matches for Ronnie Moore's side, including the Carling Cup second round tie versus Colchester United next Tuesday.
The former Wolves man has become surplus to requirements at Fratton Park, having had loan spells with Sheffield Wednesday and Walsall last season.
"Carl's a good player with bags of experience and he's sure to give us something different," revealed Moore.
"He will sit in the midfield and look after the back four but can also play in any kind of midfield role you ask him to.
"I'm still on the look out for a few more players but this is definitely a step in the right direction."
With the club third from bottom in Division One, Moore has wielded the axe by placing Branston and Monkhouse on the transfer list.
Three defeats on the trot to Wigan Athletic, Crewe Alexandra and Sheffield United have prompted Moore to make changes, with Branston and Monkhouse both likely to make exits from Millmoor in the near future.
"After three defeats it's time to grow up. We can't carry on like this otherwise we'll be dead and buried by Christmas," added Moore.
"They're both on the list and can go on loan to anyone that wants them.
"And if some club out there likes the look of them, something permanent might be arranged if they've got the dough."
