Tomlin, 26, joined Boro from Peterborough United in January 2014 and went on to score 14 goals in 64 games, including 11 in 50 last term.
The £3m deal made Boro £1.5m profit in the 18 months at the Riverside.
"I never push players to leave when the player is comfortable with the team," Karanka told BBC Tees.
"Last season Lukas Jutkiewicz came to my office asking to go to Burnley because it was an amazing opportunity.
"This season Tomo came to my office and he said he had an offer to play in the Premier League, I said it wasn't my decision, it was about the club.
"I didn't want to break his dreams to play in the Premier League. The main thing for me is the team and I want to have the players happy."
Striker Jutkiewicz joined newly-promoted Burnley in July 2014, moving up a division in the process.