Boro have lost their last four Championship games and have slipped six points adrift of Leicester, who occupy the final automatic promotion place.
Mowbray, though, will only make a loan signing if it significantly added to the strength of his squad.
He told the club's official website: "If there was a player we thought would make a difference and we could afford, we would be in the market.
"We have got enough footballers - if anything, we have got too many.
"It's no good adding another one unless he's an exceptional player who can make a difference, and generally teams are not in the business of giving exceptional players away."
The Teessiders host Barnsley on Saturday desperate to halt their recent slide.
Mowbray said: "Sometimes defeats concentrate the mind and training has been very focused and professional.
"We have been trying to get back to basics and some real hard work. I spend a lot of time looking at what we are doing and how we are training, and we have changed a few things.
"We are always looking for reasons, but it's difficult to put your finger on it.
"We have to get back to winning ways. There's not a lot wrong apart from that confidence is not where it would be if we had won the last four games."