The Teessiders finished fourth, rounding off the regular campaign with a dull 0-0 draw at home to Brighton, which secured a two-legged play-off semi-final against Brentford.
The last league game of the league campaign was very much an anti-climax in front of a bumper Riverside crowd, with the 33,381 attendance being Boro’s biggest of the season.
But the entertainment did not live up to the billing as both sides seemed happy to play out the draw.
Karanka said: “It’s not important who we play against in the play-offs, but we need to improve because the last few performances weren’t good.
“We were expecting an amazing game with the fans behind us. We have another game at home to thank our fans for this season. We have to prepare as well as we can.
“It’s not a worry – it’s frustrating for me. I always want more.
“It’s been an amazing season and we can’t forget now we have to prepare well and be more together than ever to go into the play-offs in a positive way. Promotion is in our hands.
“We’ve beaten Brentford twice, but this is different. We can’t think we’re better than them or go in over-confident. We’ve got to forget the 46 games and concentrate on the next two.
“It’s a big surprise about Derby not getting in and a very good thing for us. It shows how difficult this league is.
“I thought they’d be in the highest position and now they’re on holiday.”
Star striker Patrick Bamford was missing from the Boro line-up against Brighton with a slight injury.
Karanka said: “I hope Patrick ‘s okay for the first play-off game and I hope he can play. I didn’t want to risk him today.”
Brighton finished the season fifth-bottom of the Championship but comfortably six points clear of the relegation zone.
They might have won all three points at Boro after missing a couple of clear-cut chances.
Seagulls boss Chris Hughton said: “We were desperate to give a good account of ourselves.
“I wanted the game to mean as much to us as it did to Boro. As an away side, we did really well. We could have nicked it with a couple of late chances, and it looked like it would be one individual flash of brilliance that would win it.
“Overall a draw was a fair result and I was pleased to finish the season on a positive.”
Hughton can see any of the four play-off semi-finalists – Norwich, Boro, Brentford and Ipswich – going all the way.
“Middlesbrough have been up there all season and are a quality team,” he added. “Any of the four teams are capable of going through to win it and a lot of it depends on the day.”