Phillips, who scored 26 goals in 873 appearances for Saints between 2003 and 2005, also pointed out the defensive frailties of his old side.
They are winless in their last six Championship matches under head coach Tonda Eckert, who replaced sacked Will Still with Saints 21st in November.
Phillips acknowledged results early in Eckert’s tenure, which mean Saints are now 15th and eight points off of sixth place, but he has his doubts.
He told the Daily Echo, via BestBettingSites.co.uk: I think they’re running out of time. Do I think they’ll make the playoffs? No, if I’m being honest.
Kevin Phillips hit Premier League double figures twice for Saints (Image: Echo)
“There are probably stronger teams in the Championship, which I shouldn’t be saying, really, should I, about Southampton?
“I know them very, very well. Spent many years down there, and for me, it’s uncomfortable to see because it’s a fantastic football club.
“I think just consistency is a big key at that football club at the moment. They changed the manager and brought Still in at the beginning of the season.
“That didn’t work. I was very dubious about that decision when I saw it, having not managed in this league, one of the toughest leagues in the world.
“When the results didn’t come, it was only going to be one outcome. Then they change managers, bring another one in who got off to a good start.
“They were scoring goals when Eckert came in, and he kind of took the pressure off the players and just let them go and play. They got some results.
“But again, now they’re going back to conceding. They’re conceding too many goals. Defensively, they’re poor at defending against crosses.
“They’ve conceded 38 goals, and they’ve only scored 38 goals. So clearly, the goal scoring isn’t too bad, but conceding 38 is not good enough.
“That’s an area that they need to tighten up on if they have any chance of going on a run and making the play-offs this season.”
Saints are back in action when they host play-off chasing Hull City at St Mary’s on Saturday, January 17 (3pm). The Tigers are seventh in the league table.
Saints were beaten 3-1 at MKM Stadium earlier this season, under Still, with Adam Armstrong’s goal coming only as a consolation effort in added time.
There are 20 matches still left in the Championship campaign, with a trip to bitter rivals Portsmouth to come next weekend after facing Hull.