Mido will complete the formalities of a £6.8million move from Tottenham on Thursday to increase speculation that Yakubu will be sold.
The Nigerian has been linked with moves to Everton, West Ham and his former club Portsmouth after reportedly telling Boro boss Southgate he wants to quit the Riverside Stadium.
He was dreadful during the 1-0 defeat at Wigan but Southgate denied the speculation was beginning to affect him on the pitch.
"I don't think so," said Southgate.
"He's still with us and he's still part of what we want to do.
"Until that changes he's a very important player to us."
Mido watched from the directors box at the JJB Stadium and could be forgiven for thinking twice about joining Southgate's men.
Antoine Sibierski scored the decisive goal in the 55th minute to give Wigan boss Chris Hutchings his first win in charge of the club and leave Boro pointless after two games.
Southgate is missing key defenders at present but he nevertheless admitted his side's early-season form is worrying.
"We didn't do enough to win the game," said the former Boro skipper.
"We weren't positive enough until after they had scored - and at the moment I don't know the reasons for that."
New-look Wigan embarrassed Boro at times and Sibierski's predatory strike was the least they deserved for their superiority.
Hutchings said: "It's nice to get the first three points on the board, but it's only one game.
"But it is vital to win your home games and I'm delighted that we have started in the right manner."