Emnes made his first start for Boro in over a year in the 2-2 draw with Hull City on Saturday, with manager Tony Mowbray keen to give him an opportunity to prove himself.
The Dutch forward, 22, was discarded by previous manager Gordon Strachan, who had tried during the summer to offload him.
But McDonald thinks Emnes has shown a determination to prove himself.
McDonald said: Marvin is hungry and wants to achieve things. He just wants to play and I've got to give him his due because over the point of time he has not been involved in the squad he has been so professional.
I don't know what I would have done had that been me.
I would have gone out of my mind, he held it together and kept wanting to work at it. He got his chance at Swansea and delivered.
Emnes, a £3.2m signing from Sparta Rotterdam and a Holland Under-21 international, had not started for Middlesbrough since October 31 last year Strachan's first match in charge.
His five-match loan at Swansea proved enough to convince his new manager that he was worth a go.
Marvin is only going to grow the longer he plays with us and for us. He has been out of this team for a while, so it will take time for him to get in to the rhythm, said Mc- Donald, ahead of this Saturday's trip to Coventry.
There is no doubt he has quality, that's why the club bought him in the first place.
He is only young, learning and he has goals in him as well. Hopefully he will provide a few goals for the rest of us as well.
A signing session with Luke Williams, Julio Arca and Andrew Halliday that was due to take place at Middlesbrough's official store this evening has had to be cancelled because of the weather.