The Australia international netted 64 goals in two and a half seasons at Celtic Park but Gordon Strachan signed the player in January for a fee of around £3.5m as Tony Mowbray reshaped his squad.
Mowbray has since been sacked and replaced in the short-term by Neil Lennon and McDonald would not rule out going back to Parkhead one day.
That's entirely not up to me,'' McDonald said in an interview to be screened on STV's Sports Centre tonight.
Football is a funny old game though, never rule it out. To play for Celtic again would be unbelievable.
It was a tough decision to leave but I felt it was time to move on when you're not really on the same wavelength as the manager.'' McDonald has scored three goals in an injury-affected period at Middlesbrough, whose play-off hopes ended with defeat at West Brom last weekend.