The former Watford striker has been training with Boro this week, after his truncated loan at Hull, and Southgate said: "I've been absolutely delighted with him so far.
"He's got a hunger to be here and I think he has a point to prove.
"The great thing from the squad's point of view is that it's another striking option. Marlon has Premier League experience and he is fit from his time at Hull City, so we can put him straight in if we need to.
"In many respects I'm delighted that things didn't work out at Hull for him because it means we have got him here.
"A new player coming into a squad always makes a difference, especially when he is a goalscorer.
"It lifts the other players and they are keen to show him what they can do as well.
"So I am hopeful that Marlon's arrival will work very well for us.
"He knows what he are looking for from him and the importance of our situation and he is keen to try to make a difference."