Mowbray was in the Celtic Park hotseat when McDonald and Robson were sold to Boro during the January transfer window, while McManus joined on loan before a permanent switch to the Riverside Stadium in the summer.
The former Celtic, West Brom and Hibernian manager said: "The fact is Gordon had worked with some of them and seen what professionals they are - and they are good professionals - but they weren't playing for me, the opportunity was there and it suited all parties at the time so those boys know that there was no fall-out with the manager or no animosity between us."
With predecessor Gordon Strachan also returning to Scotland to sign Kevin Thomson and Kris Boyd from Rangers during the close-season, Boro were installed as favourites to win the npower Championship title and return to the Barclays Premier League at the second attempt.
However, Strachan quit with the club 20th in the table - their lowest league position for 20 years - while successive 1-0 defeats at Nottingham Forest and Norwich have since seen them slip to third from bottom. Their disappointing start to the campaign comes after last season's unimpressive 11th-placed finish on the back of their relegation from the top flight in 2008/09.
Mowbray has now been charged with the task of turning his hometown club around but he has stressed the whole squad will start with a clean slate and he is convinced McManus, McDonald, Robson, Boyd and Thomson can cut it at Championship level despite not pulling up any trees as yet.
"They were all sitting in the front row when I had a meeting with them, looking me in the eye and hopefully looking forward to it and getting an opportunity. They will all get the same opportunities. The guy who has played one game for Middlesbrough, a young guy coming through, the seasoned professionals, it all starts level.
"All I ask them to do is to come in every day and work hard, be good professionals and try and help the team be successful, try and show the supporters they care for this team and together we can be successful.
"I have worked in both the SPL and the Championship and the likes of Thomson or Boyd or McDonald or Robson are more than capable of playing in this division.
"Sometimes there is a transitional period, a settling in period, things that people don't see, the fact they are moving the family, maybe moving a child into a new school, getting a new house. They are talented boys and driven footballers and I know Gordon liked them because of their character and personality, all those things are positives for me."
Source: PA
Source: PA