A number of Glasgow's squad will move on this summer, including stand-off Finn Russell who is headed for Racing 92, but Rennie says that their replacements are already in place.
Speaking after Friday's PRO14 exit at the hands of Scarlets, Rennie told The Telegraph: "We absolutely need to strengthen for next year but the big thing is around the go-forward. Our scrum is reasonably good, but we've got a few men to come in that will give us a bit more of an edge.
"We've done most of our recruitment but we haven't announced it yet. We've got eight or nine guys leaving and we've probably replaced about seven of those. Looking at our squad for next year, we'll have more depth."
Rennie said the performance of his young players this season augurs really well for the next campaign. George Horne has been the side's standout player this season, and was the Warriors' most lively performer against Scarlets, while teenage flanker Matt Fagerson also impressed.
"George Horne and Adam Hastings have been really impressive this season," said Rennie. "We talked a lot about Adam with regards to the last couple of games because he's been really impressive in training, and has been running our opposition a lot. I thought he played really well against Edinburgh.
"Matt Fagerson is only 19 and is still a kid so he'll get better and better because he's still got a fair bit to learn. Then we've got a few young guys coming through in the tight five and some firepower in the backs, so I guess we've got the makings of a good side and when we go to pre-season this year we won't be starting from scratch."
The priority now, says Rennie, is for the players to get a decent month-long rest after a particularly long season. Although many will tour with Scotland, some key players have been given time off to rest tired minds and bodies.
"Because Glasgow didn't make the play-offs last year the boys were in on June 1 so they've been going for 50 weeks and I think they need to mentally freshen up," said Rennie. "Physically too, they need to get away. They'll get tested when they get back and there's an expectation that they will be in bloody good nick, but it's important that they get away, clear the head and get excited about next season."