The Flames said the 19-year-old Monahan's status is week to week.
The team also announced Thursday that Wideman fractured his left hand in the previous night's 3-2 loss to the Chicago Blackhawks. Both players have been placed on the team's injured reserve list.
Wideman will undergo surgery to have a pin placed in the hand and will be out six to eight weeks, according to general manager Jay Feaster.
Calgary (8-12-4) is on the road in Anaheim on Friday and in Los Angeles on Sunday.
With nine goals and six assists, Monahan is second in NHL rookie scoring behind Thomas Hertl of the San Jose Sharks. Monahan is also second in Flames scoring.
He took a shot off the foot in practice Nov. 21, but played in Calgary's next two games. Monahan scored his ninth goal of the season against the Blackhawks, but his foot continued to be painful. A scan Thursday confirmed the fracture.
"He will be re-evaluated at the beginning of next week and we'll have a better idea exactly what the time frame will be," Feaster told reporters Thursday.
"It's certainly not good news when these things happen. To lose a guy like Wideman that's a tough blow for the team. Obviously with Monny, the way Monny's been playing, it's a tough guy to take out of your lineup."
The injury bug has bitten the Flames hard this season. Captain and top defenceman Mark Giordano hasn't played since he broke his ankle during a game Oct. 21. He has started skating again and Feaster is hopeful the captain will be ready to play next week.
Forward Curtis Glencross is out two to three weeks with a knee injury and forward T.J. Galiardi is day to day with back spasms.
"We've lost some significant players for a significant period of time," Feaster said. "It's important they don't become an excuse. They're not a crutch, no pun intended, to lean on.
"You try and find a way to soldier on. We have to find ways to win games with the guys we do have."
The Flames have called up forward Max Reinhart and Paul Byron from their American Hockey League affiliate in Abbotsford, B.C.
Flames head coach Bob Hartley said that Reto Berra will start in net against the Ducks on Friday and Karri Ramo will play Sunday against the Kings.