Fleury is 25-14-5 in his third season with Vegas and has won his past four starts, but his 2.79 GAA and .906 save percentage are his lowest since 2016-17 with the Pittsburgh Penguins.
Subban was 9-7-3 with a 3.18 GAA and .890 save percentage as Fleury's primary backup this season.
Fleury said he believes Lehner can push him to be better in the final 18 games. Vegas (34-22-8), on a season-high six-game winning streak, leads Edmonton and the Vancouver Canucks by two points for first place in the Pacific Division.
"I think it pushes you to do well, to keep playing," Fleury said. "It's only good for the team also. If you can push each other to do well, the team will benefit from that."
Fleury, who has started 151 games the past three season, said he would welcome a split workload with Lehner, and that he's committed to doing whatever is best for the Golden Knights. Lehner said his own workload is "not on my agenda."
Vegas coach Peter DeBoer said he plans on rotating the two.
"There's no doubt we have two elite goalies," DeBoer said. "I'm a big believer that competition at any position is a great motivational tool and it pushes competitive people to new levels. I'm excited about that."
Fleury said adding Lehner is a win-now sign for the Golden Knights, who are trying to reach the Stanley Cup Playoffs for a third consecutive season and the Stanley Cup Final for the second time in their three-season history.
"That's what we play for," Lehner said. "I think this team has a legitimate chance. Really good team, good coaching staff, they have all the ingredients."