Doku has missed City's opening three Premier League games with a calf injury picked up in last month's Community Shield defeat to Arsenal, while O'Reilly withdrew from the starting lineup in the warm-up before Saturday's 1-0 win over Coventry with a back problem, handing deadline-day signing Enzo Fernández an unexpected debut.
"Nico probably yes, Jeremy we're not sure yet, we need to wait," Maresca told reporters when asked whether the pair could feature against Manchester United.
Pressed on whether Doku had suffered any setback in his recovery, the head coach added: "No. He is back. I don't know if he can be available for the weekend, but slowly, slowly he is back."
Neither player has travelled with the squad for Tuesday's Champions League opener at Porto, meaning City's first game under Maresca in the competition will be played without two first-team regulars from last season. O'Reilly's absence is expected to be brief given the encouraging update, while Doku's recovery timeline remains less certain after a spell out that has already stretched beyond the club's initial two-to-three week estimate.
Manchester City head to Estádio do Dragão on the back of three wins from three in the league, and Maresca will be keen to make it four from four across all competitions before turning his attention to Sunday's trip to Old Trafford, his first Manchester derby in charge.
Should both Doku and O'Reilly remain unavailable for Porto, Maresca has options elsewhere in his squad to cover the flanks, with Rayan Cherki, Phil Foden and Matheus Nunes among those who travelled to Portugal in a 22-man squad.
Doku has been one of City's most effective attacking players over the past two seasons, and his absence has been keenly felt during a season in which Maresca has continued to rely heavily on pace out wide.
O'Reilly, meanwhile, has established himself as a regular under Maresca since breaking through from the academy last season, operating comfortably at both left-back and further forward, and his versatility makes his potential return for the derby particularly valuable.