The 20-year-old netted his third goal in four starts for the Hornets at Scunthorpe on Saturday and has become a crowd favourite at Vicarage Road.
And with England midfielder Stewart Downing currently keeping Johnson out of the Boro team, Southgate is sure he will benefit from regular starts at the Championship club.
He said: "We have to take the long-term view of it and we knew when we sent him away we might be short of numbers at some point, but Stewy is playing well and we're ok in the wide areas.
"Long-term it would be of more benefit for him to gain the experience he's getting.
"We've sent him down there because he's very much part of our future."