Australian striker McDonald and Scottish centre-back McManus both arrived on Teesside from Celtic in 2010 but could be shipped out by Boro who are trying desperately to cut their wage bill.
Mowbray has indicated that the duo demand a fairly high salary but says that so far no clubs have enquired about their availability suggesting they could remain at the Riverside Stadium.
"There have been no phone calls for either of them," he told the Evening Gazette.
"I fully expect all the players to come in, get their heads down and work hard to try to get in my plans for the season.
"Players on large salaries here now - just as players on large salaries who were here last year - you get on with them, it's your job.
"You hope they are good enough to impact on the team and make a difference and we hope that's the case this year."