The 25-year-old joined the Riverside club in the January transfer window, and within the first six matches into his half-season loan, he has already made a huge impact.
Boro have only lost one of those matches he has started in since moving from Goodison Park, and Tony Pulis is now considering making a permanent move for the Bosnian.
He has three years to run on his present contract and Everton will be looking to get more than the £4.