Deacon, who came up through the Tigers' ranks, returns to Welford Road after two years at Gloucester. The 30-year-old arrives as an injury dispensation signing following long-term setbacks for Tom Croft (neck) and Craig Newby (knee).
Australian Phibbs, also 30, arrives at the club after injuries to Ben Youngs (shoulder) and academy scrum-half Scott Steele (ankle). He has Super 15 experience for the Brumbies and played for Exeter last term.
"Brett has done well for Gloucester during the past couple of years and he already knows the environment well at Leicester," Tigers director of rugby Richard Cockerill said.
"We have had some injury issues in the back row with Craig Newby and Tom Croft out, and Jordan Crane coming back from long-term injury.
"Brett has played more than 100 first-class games and can come in and fit in where we need him to because he has plenty of experience.
"At scrum-half, Ben Youngs is out after shoulder surgery and Scott Steele has not done any pre-season because of the injury he picked up at the end of last season.
"He will not play until the end of August so we need to have a bit of an insurance policy at scrum-half.
"Pat is that person and will be here for four months.
"Matt O'Connor knows him well from his time at the Brumbies, and Pat knows the Aviva Premiership well because he was with the Exeter Chiefs last season."