HAWTHORN defender Grant Birchall will miss at least four to six weeks after damaging the posterior cruciate ligament in his right knee against West Coast on Friday night.

The All Australian half-back suffered the injury during the first quarter of his team's 20-point win, but played on until the third quarter after Hawks medical staff assessed the problem as a corked calf.

Hawthorn head of coaching and development Chris Fagan said Birchall was sent for scans when he arrived at recovery with a swollen knee on Saturday.

"From those scans, we've discovered that in the same incident where he hurt his calf, he also hurt the PCL in his knee when he hit the ground," Fagan told hawthornfc.com.au.

"Unfortunately that means he's going to be out for somewhere between four and six weeks, and it could even be longer, depending on how his rehabilitation progresses."

Birchall has played 11 of 12 games this season, averaging 23.5 disposals per game.

He will now be out of action until at least the Hawks' round 18 clash with Essendon, with the likelihood he will need until round 20 or beyond to regain full fitness.

His loss adds further pressure to a backline already missing playmaker Matt Suckling and key defender Ryan Schoenmakers, who have both had anterior cruciate ligament reconstructions.

The outlook is better for fellow defender Josh Gibson, who has escaped damage to the ankle he rolled late in the win over the Eagles.

"He (Gibson) was sore after the game, but he's made really good progress in the early part of this week," Fagan said.

"There's no serious damage to his ankle, so if he trains well, he should play."

Small forward Paul Puopolo, a late withdrawal on Friday night with quad tightness, will also be tested closer to selection.

"He needs to get through the week fit and healthy before we select him on the weekend," Fagan said.

Isaac Smith, who missed last week with a corked calf, will be available.