Williams' last senior appearance came in the penultimate game of last season, with a serious pelvic injury ruling him out both last summer's World Cup finals and the majority of the current campaign.
He stepped up his rehabilitation with an 84-minute appearance in Wednesday's reserves defeat against Hartlepool, and while he will not be in the starting line-up for tomorrow's Championship game at Reading, he is set to be named on the substitutes' bench at the Madejski Stadium.
"Rhys will be in the squad that goes down to Reading," said Mowbray, who has been without a host of senior players for the majority of his time at Middlesbrough. "He's played three reserve games now, and although he hasn't finished any of them, it's a progression that's going well.
"I'll talk to him to see how he feels, and I'll see what the rest of the squad is like in terms of numbers, but it's about time he got back involved with the first team. Over the next few weeks, I'm sure he'll get some game time off the bench."
That game time could come in a variety of positions, but Boro's current shortage of centre-halves means he could spend the final two months of the season covering in a central defensive role.
Despite starting his career as a defender, Williams has never played in a central defensive position for Boro's first team.
He spent half a season on loan at Burnley playing at right-back, and appeared as a central midfielder under both Gareth Southgate and Gordon Strachan prior to Mowbray's arrival.
However, with Stephen McManus, Matthew Bates and Andrew Davies currently sidelined, the Boro boss is keen to see whether Williams can prosper as a centre-half.
"If we need Rhys in a certain position, we'll have to utilise the versatility he has," said Mowbray. "I know he played a lot of football for Burnley at right-back, but I've looked at our centre-half situation and seen a boy who's six foot two, very athletic and very competitive.
"He looks to me as if he could play at centre-half all day long. He's played centre-half in a reserves game and a behind-closed-doors match and done well.
"We will utilise him where we feel necessary, but he's been out a long time and it's never easy just to flick a switch and get back to being the player you know you can be. We'll be looking to ease him back in."
Williams' return is particularly timely given the ongoing problems afflicting the Boro defence.
McManus and Davies remain two weeks away from a first-team return, while Bates is set to be sidelined for at least another month.
"Stephen's been doing some training with the youth team this week, so it'll probably be another week until he trains with the first team and then he'll need a reserve game under his belt," said Mowbray.
"Andrew's two or three weeks away as well. I don't think it's a severe hamstring injury, but we'll have to see how he progresses. It's a blow to him because he's come here to try and play some football and rebuild his career. One and half games in and he's found himself injured."
Bates is also rebuilding his career after a series of lengthy lay-offs, and while his current hamstring problem is not related to the four cruciate ligament injuries he has sustained in the past, Mowbray has given the Boro skipper permission to travel to America to visit knee specialist Bill Knowles, who oversaw his recuperation last summer.
"Mentally and psychologically, when you have a number of long-term injuries, you inevitably look to the guy who has got you through them best," he said. "If Matthew believes in him 100 per cent and has seen the benefits of working with him, then it's only natural that he wants to see him again.
"I have absolutely no problem with our medical department allowing Matthew to go back to the guy he has faith and trust in. He goes to America with my blessing, and hopefully he'll come back in a few weeks not too far away from being ready to go again."