The Tannadice club have accepted an offer from the Black Cats for the 17-year-old, who has made 14 senior appearances for the Terrors.
He made his debut in January this year in the 2-2 draw with Aberdeen and has impressed this term after deputising for the suspended Gavin Gunning.
But United manager McNamara believes the teenager should rack up at least 100 games before heading south.
He told STV: "The latest (with John's situation) is that he is still a Dundee United player. The club accepted another bid from Sunderland and he went down to see the facilities.
"I think it is important now that we have that dialogue with John. If that is what he chooses to do (join Sunderland) then we will never stand in anybody's way.
"But if you are asking my opinion then I think he would be best to stay here for the next couple of years and get a few games under his belt.
"It will be his decision. He has just turned 17, it is all about his development. He is part of our first-team squad at such a young age and it would be good for him to play 100 or 200 games first and get that experience."
Despite the speculation surrounding Souttar, McNamara is happy to have him in his squad for Saturday's match with Kilmarnock at Tannadice.
He said: "If anything I think it (the speculation) will give him extra confidence. He's quite a level-headed young lad and he has a good family behind him. He won't be fazed by anything."
Meanwhile, McNamara has said that he is keen to bring Morgaro Gomis back to Tannadice after he impressed while training with the club.
Midfielder Gomis, who was previously on the books at Tannadice between 2007 and 2011, is a free agent and available to sign outwith the transfer window but finding the cash to seal his return is now the biggest issue facing United.
McNamara said: "Morgaro has been training for the last month with us and he played a closed-door game last week. He was very impressive.
"Like most clubs, budgets are done at pre-season time before the window closes but if there's something that can be done with Morgaro then we'll look at that. It could come down to finances."