A deadline for the Teesside club to take up an option on a permanent deal for Vossen expired on Tuesday and they now have to weigh up whether to make a firm £2.3m offer before the summer, but Karanka will not be pressurised.
He said: “The situation is that I am thinking about the (Wigan) game, Jelle is thinking about the game and anything else we will see.
“I don’t think Jelle is thinking about what will happen in May.
“We are focused on the game. We have to decide at the end of the season.”
While the odds may suggest a home win today, with Boro one point off top spot and third-from-bottom Wigan fighting for their lives at the bottom, Karanka is wary and also mindful of the Latics’ fine recent away form, which has seen them pick up three wins on the spin.
He added: “We have one month to go and it’s very exciting.
We can’t make mistakes, and the biggest mistake is thinking it will be easy.
“Wigan are playing to stay in the league and they are playing better away from home. It will be a difficult game. They defend very well and it’s difficult to score against them.
“It is a surprise Wigan are where they are.
“I said after our game in November that I expected Wigan to go up the table because that day they were very organised.
“I know Malky Mackay does a good job, look at what he did at Cardiff, and we know it will be a difficult game.”
Meanwhile, Patrick Bamford has been passed fit to start for Boro, despite having to withdraw from England Under-21s’ squad last week.
Karanka said: “Patrick had a problem with one of his feet, but now he is better. We had a lot of games in February, but the doctors here and the FA agreed it was the best way for him to recover.
“He was recovering last week, but he has trained with us this week.”
New loan signing Fernando Amorebieta will be assessed after his return from international duty with Venezuela, while Karanka has confirmed that fellow loanee Tomas Kalas will leave the club after the away game at Norwich City on April 22.
Karanka revealed that Ryan Fredericks is back at Boro following his spell at parent club Tottenham spent recuperating from a hamstring injury.
Last six: Middlesbrough LWLWWL, Wigan WWLWLD.
Referee: S Attwell (Warwicks).
Last time: Middlesbrough 0 Wigan 0; January 28, 2014; Championship.