The targetman missed Saturday’s 2-0 defeat at Brentford after pulling out late complaining of a tweaked groin.
City were also without Andrew Davies, who felt his calf problem was not quite ready for Griffin Park.
But it was Hanson’s absence that hit City the hardest against the third-placed Bees.
Parkinson had worked on Hanson playing the lone front role with Chris Atkinson, making his first start, in support. But the late switch meant Aaron Mclean was thrown into the position.
Parkinson said: “It’s frustrating because we’d worked on a system with Hanson and Atkinson in behind.
“Hanson pulling out changed the dynamics of the team a lot. He’s involved in a lot of our play.
“What we’d set out to do worked to a degree but obviously Hanson and Mclean are different players. Aaron worked really hard but it was a tough job for him.
“We’d set out to work with Hanson and Chris and they’d looked good. Hanson has got that ability to stretch the pitch on the angle as well.”
Parkinson will give Hanson and Davies as long as possible before deciding if they will face Colchester – the first of three games now against teams below City.
The City boss said: “I don’t know if they will be ready, we’ll have to see. But it is disappointing. When you come to a team like Brentford, you need all your big players out there.
“It was similar to the Wolves game. If it had been the last day of the season, then possibly we’d have thrown those two out there, but we just couldn’t take the risk.”
City’s hopes of frustrating Brentford lasted an hour before they were undone by Clayton Donaldson. On-loan Chelsea midfielder George Saville then added a second goal.
But after the two late Stevenage strikes at Valley Parade last week, Parkinson was annoyed that his side were again beaten by a goal from outside the box.
He said: “We’d spoken to the lads about getting out to block shots. It’s annoying that three goals now have all been scored from distance.
“All those players have got good ability and we’d done great up to that point. But time almost stood still in that one moment and everyone on the bench was shouting ‘get out to the ball’.
“We just needed to block it, or at least to get close and make it more difficult for Donaldson. It was just too easy.
“We looked okay up to that point. We were solid and compact and everyone was doing their job. We’d competed well for an hour. But when the first goal went in, we gave a good team a huge lift.”
Loan defender Adam Drury came off the bench in the closing minutes. Parkinson had decided against playing the Leeds left back from the start because of his lack of senior match practice.
The City chief said: “He’s had some under-21 games and to get a left back who’s been playing regular first-team football is almost impossible.
“We possibly could have taken a couple of youngsters who’ve never played league football before.
“But we’ve got a couple of those ourselves and anyone coming in on loan has got to be a lot better than them to justify bringing one in. Adam’s vastly experienced.”