Newcastle Jets coach Scott Miller looks set to face disruptions with his back-line for at least another four weeks because of a knee injury to Daniel Mullen.
Mullen, the Jets' preferred right-side centre-back, missed the 2-1 loss to Western Sydney Wanderers on Saturday evening at Hunter Stadium after injuring his knee in a challenge at training last Monday.
Miller revealed after the loss that Mullen was expected to miss four to six weeks of action.
Former Roar defender Lachlan Jackson came in for Mullen against Wanderers and played left-side centre-back, pushing skipper Nigel Boogaard to the right.
The 20-year-old Jackson, who has just five A-League games under his belt, played two weeks earlier, in the 1-0 win over Melbourne Victory, at the left of the back four.
Although Wanderers scored their comeback win via a 79th-minute Andreu penalty given away by striker Milos Trifunovic and an 86th-minute long-range strike from Mitch Nichols, the Jets were more more vulnerable at the back without Mullen.
The defensive effort continued a disrupted start for the Jets back-line over the opening five rounds.
Miller has had his preferred back four of Lee, Boogaard, Mullen and Jason Hoffman just twice, in wins over Wellington and Melbourne City.
After the loss to the Wanderers, Miller admitted the absence of Mullen was a problem for the Jets.
"It's not only just Lachlan, it changes the balance of the entire team," Miller said.
"Lachlan just needs to really bed-in and build on his performance this evening and just really play within the structure of the team.
"Lachlan will continue to progress, he's got self-belief, which is warranted, and he will progress.
"But it does change a lot of the structures.
"It changes the relationship with Ki-je and Nigel (Boogaard), who have worked well together, but they'll just build momentum as we go."
Enver Alivodic had put Newcastle ahead of Western Sydney in the 73rd minute despite appearing offside from a Lee free kick which rebounded off the crossbar.
The Jets, third before the weekend, remain on nine points with three wins and host Central Coast Mariners next Saturday in round six.
The Wanderers moved to seven points and next face Melbourne City away on Friday night.