Lee rejected the opportunity to return to his former club Pohang Steelers on transfer deadline day, but looks certain to depart Teesside over the summer.
Having become a target for abuse from certain sections of the Boro faithful, Southgate confirmed that injuries to a number of his more favoured front men would be the only way in which Lee would be welcomed back into the fold.
"Obviously he hasn't been in the first-team squad for a little while," Southgate told the Middlesbrough Evening Gazette.
"I think it's been difficult for him in that I think he has not played as well as we might have hoped.
"But I think he has become a bit of a figure of getting a lot of stick and it's very difficult, especially at home, when I put him on.
"I think it's difficult for him to have any sort of a chance.
"For him and for the team I think it's been better to involve other players really.
"As for him playing again this season it depends on how the other strikers are doing really.
"We've obviously only got five games to go and I'm hopeful that the strikers we've got perform between now and the end of the season and score the goals that we want them to."
