Both golfers finished on 69 with near-identical rounds as they sit in a 10-way tie for sixth as the tournament reached the halfway stage.
With fellow Italian Francesco Molinari able to follow up his first round 62 with "only" a 70, Manassero's scintillating seven birdies in the last nine holes brought him only one off the lead in Inverness. Molinari had a triple bogey seven on the 464-yard seventh to allow Swede Alex Noren to join him out in front after a second successive 66 the day after his 30th birthday.
Argentina's big-hitting Ricardo Gonzalez is joint third with Manassero, while world number one and defending champion Donald's 68 means he has only three shots to make up and Phil Mickelson's 64 brought him just five back.
Manassero, the Tour's youngest-ever champion could qualify for Royal Lytham next week with a top five finish, but his sights are on emulating what German Marcel Siem did at the French Open last Sunday by winning to earn his spot.
"The Open is on my mind and this is my last chance," said the 19-year-old from Verona, who stands 11-under-par at halfway.
Lewis, the amateur star of last year's Open, will not be at Lytham after his miserable recent run continued with rounds of 73 and 72 for one-over-par. Montgomerie finished one under and Lawrie two under, but Casey looks no nearer finding his old form.