Thrashers coach/GM Don Waddell said Kovalchuk, who is nursing an injured right knee, will likely miss his third straight game and is listed as day-to-day. The Thrashers are 2-0-1 without Kovalchuk this season.
"He's getting much better than he was before the weekend," Waddell was quoted as telling local reporters in the Atlanta Journal-Constitution. "I think our doctors would be more comfortable if [he didn't play]. I think he's out."
Waddell added that Kovalchuk was probable for Thursday's game against Vancouver.
The Flyers, meanwhile, will be looking for their eighth straight victory over the Thrashers when the teams meet on Tuesday night.
Philadelphia (29-17-5) has not lost to Atlanta since a 6-5 overtime loss at home on Nov. 18, 2005. The Flyers have not lost in regulation to the Thrashers since a 4-1 defeat in Atlanta on Dec. 21, 2003, going 11-0-1 since then.
Since the Thrashers (26-25-4) entered the league in 1999, the Flyers are 23-3-1 with three ties against them, including a 12-2-1 mark at Philips Arena. Philadelphia has won the first two meetings this season by a combined score of 8-1.
"When you have a team's number, it gives you confidence," said Philadelphia's Scott Hartnell, who had a goal and two assists in the most recent matchup, a 4-1 win in Atlanta on Jan. 8. "You play with that confidence on the ice."
That win was part of a 13-3-1 run that has catapulted the Flyers from last place in the Atlantic Division into a three-way tie for first with New Jersey and Pittsburgh. After 706 career games, Mike Knuble recorded his first NHL hat trick in Saturday's 3-0 win over defending Stanley Cup champion Anaheim.
However, the coach will be switching goalies for the game, as Antero Niittymaki will get the nod over Martin Biron, who made 27 saves for his third shutout of the season on Saturday. Niittymaki is 7-0-0 with a 1.58 goals-against average in seven career games against the Thrashers and had 28 saves on Jan. 8.
"I know [Biron] is coming off of a shutout, but looking at the schedule, I kind of targeted that game for Niitty only because it seems to bring out his best game," Stevens told the Flyers' official Web site. "We're going to need a great game from Niitty. They are playing well, and it allows Marty to get right back in the net at home."
The Thrashers have won three straight, one shy of their season high. Todd White broke a scoreless tie and Marian Hossa added an empty-net goal in Saturday's 2-0 win in Washington, helping Atlanta move into a first-place tie with Carolina in the Southeast Division.
Kari Lehtonen made 36 saves in his third shutout of the season, helping Atlanta win despite being without Kovalchuk, who leads the team with 38 goals and 64 points.
"We didn't want to get in a shootout tonight," coach Don Waddell said. "We didn't think that would be to our advantage. We really wanted to keep it tight. When you're missing your leading goal scorer, it's always a challenge. You have to change your game plan to play to your strengths."
Information from STATS LLC contributed to this report.