Defender Alcock came through a 90-minute run-out for the development squad this week having struggled with a knee problem recently.
On-loan Manchester United midfielder Petrucci missed the 5-1 win over Millwall in midweek due to a hamstring injury.
Posh boss Darren Ferguson had probably hoped to name an unchanged team following Tuesday night's performance at The Den, but winger George Boyd moved to Hull on loan on Thursday.
Teenage left-back Kgosi Ntlhe made his long-awaited comeback from injury in the development squad.
South African Ntlhe, sidelined since suffering a broken ankle against Sheffield Wednesday on November 3, played 45 minutes of a friendly against Stevenage on Wednesday afternoon.
He will play for the reserves again next week with a view to being back in first-team contention for the trip to Blackburn on March 2.