Steele is facing up to two months on the sidelines after tearing a stomach muscle during Saturday's 5-2 defeat at Reading, leaving Boro completely bereft of experienced goalkeeping cover.
Mowbray has attempted to re-sign former Boro goalkeepers Ross Turnbull and Brad Jones, who are warming the bench at Chelsea and Liverpool respectively, and has also spoken to Blackburn about youngster Jake Kean, who spent the first half of the season on loan at Hartlepool.
The Boro boss has also considered offering a short-term deal to the out-of-contract Roy Carroll, but has so far been unable to secure an experienced alternative to Ripley, who made his senior debut as a substitute at the Madejski Stadium.
Mowbray has until 4.45pm to sign a keeper who can be involved in this evening's game with the Rams, but with time running out, it looks increasingly likely that Ripley will be charged with the task of protecting a back four that has shipped 11 goals in the last four matches.
"Connor is a very talented young goalkeeper," said the Boro boss. "He's only a boy, but he has all the attributes to be a good keeper. In an ideal scenario though, you don't throw a goalkeeper in this early in his career.
"We've been searching extensively to try to get a more experienced goalkeeper in on loan. We've been in touch with just about every Premier League club now, but unfortunately there was a flurry of Premier League goalkeepers leaving on loan last week and we're paying the price for that.
"It's one of those unfortunate situations where clubs are sympathetic to our plight, but can't do anything to help. We tried to sort it out on Saturday night, and thought we had someone sorted on Sunday. We got pretty close, but unfortunately it fell through."
Having exhausted the domestic options, Mowbray, who will also be without Danny Coyne for up to a month, was last night turning his attention to the foreign market.
Former Sweden international Jonas Sandqvist is understood to have offered his services to the Teessiders, but Mowbray is understandably reluctant to sign a goalkeeper he has never seen.
However, with Ripley having only turned 18 last month, the Boro boss is aware of the risks involved in subjecting the teenager to such a pressurised situation at such at embryonic stage of his career.
"I have a responsibility to Connor because, if he goes in and doesn't do so well, it could put him back a little bit," said Mowbray. "He's basically just a boy. He conceded three goals at the weekend, and even though they weren't his fault, you don't want to get your career off on the wrong foot.
"He's a young guy with bags of potential and has lots of assets to be a top goalkeeper, but I wouldn't want to put him in unless I had to.
"And if we don't get a goalkeeper in, then beyond Connor, we've only really got two 16-year-old schoolboys who come in to help us train.
"There are a few foreign keepers who might be able to come in, but I wouldn't bring a foreign goalkeeper in who I don't know and stick them straight in the team. The cut off is three hours before kick-off though, so we'll keep trying."
While Mowbray will continue to pursue a goalkeeper this morning, he has ruled out the loan capture of an experienced centre-half despite Saturday's defensive capitulation at Reading.
Jonathan Grounds is expected to continue to partner Seb Hines tonight, but both Andrew Davies and Stephen McManus should be available in two weeks time.
"If we were to bring another loan defender in, by another two or three weeks, they might not even be able to get on the bench," said Mowbray.
"If there was a superstar centre-half out there who was going to make a massive difference to our team, I would ask the question. But unless we're suddenly able to pay £40,000-a-week, there isn't."
Julio Arca will not be involved this evening, but striker Scott McDonald should be back in the squad after missing Saturday's game because of an illness.
"What we're finding with the young players is inconsistency," admitted Mowbray. "I know they're capable of playing really well, but somewhere down the line, I also know they're capable of making a mistake.
"In my opinion, that's why we're pretty good one week, then in the next game, somebody does something stupid and it costs us a goal. Sometimes, you get that with young, inexperienced players. It's the inconsistency of youth."