Bamford joined Boro on a permanent deal from Chelsea earlier this month for a fee of £6m.
Karanka said: “Teams in our position are signing players for £14m – we are signing players that didn’t play in the Championship.
“Patrick didn’t play for almost 18 months so I don’t think it is an amazing improvement.
“The people who know Patrick know he is not the same as he was 18 months ago.
“Anyone who thinks he is going to score five goals straight away is making a big mistake because he is not ready now.”