Senators centre Peter Regin is sidelined for the fourth time this year with a left shoulder injury, this one suffered in Tuesday's 4-1 victory over the Buffalo Sabres at Scotiabank Place (Andre Ringuette/NHLI via Getty Images).

The nightmare seems never ending for Peter Regin.

One more time, the Senators centre's troublesome left shoulder has sent him to the sideline, with head coach Paul MacLean using the word "indefinite' to describe the length of his latest absence from the Ottawa lineup.

"it's very frustrating for Peter," MacLean said in revealing the news following the Senators' pre-game skate in advance of tonight's matchup with the Florida Panthers at Scotiabank Place (7:30 p.m., Sportsnet Sens, Team 1200). "Peter's an important player we think a lot of, but it's devastating for him. Now he has to make the decision about what's best for him at this point."

"(Surgery) is one of the options, but I don't know what the other ones are or what he has to consider."

Regin's latest injury occurred on Tuesday night during the Senators' 4-1 triumph over the Buffalo Sabres at Scotiabank Place. The shoulder "stiffened up on him" yesterday, MacLean said, with the centre in need of re-evaluation by doctors before deciding how to proceed.

The shoulder is the same one Regin injured back on Feb. 19, when he tumbled awkwardly into the boards during a game against the Toronto Maple Leafs. That required season-ending surgery and several months of rehabilitation, but the 25-year-old native of Herning, Denmark, spoke with enthusiasm at the end of August about how we'll he'd recovered.

"It was my first major surgery, but it turned out great," he told OttawaSenators.com at the time. "I'm happy that I did it. (The shoulder) has been bothering me for two years now, so it was finally nice to get it done. Now it feels as good as it did before."

However, that good feeling barely lasted a month, as Regin re-aggravated the injury during a Sept. 27 pre-season game against the Leafs. He returned for the Senators' home opener versus the Minnesota Wild on Oct. 11, but reinjured it again nine days later in a home matchup with the Winnipeg Jets. After being sidelined for 22 games, Regin got back into action Dec. 10 against the Vancouver Canucks, but his latest return lasted a mere five games.

In 10 games with the Senators this season, Regin recorded two goals and two assists.

Meanwhile, the wait for Milan Michalek's return to the lineup will have to wait until after Christmas. The Czech winger, who was leading the National Hockey League with 19 goals before suffering a concussion in a collision with Ottawa defenceman Erik Karlsson on Dec. 13 in Buffalo, said today his fitness level isn't where he wants it to be just yet.

"I feel pretty good," said Michalek after practising for a second straight day in a grey no-contact jersey. "But I'm going to take an extra few days to work on my conditioning. I want to be 100 per cent ready before I come back."

That means the earliest Michalek could be back is next Tuesday, when the Montreal Canadiens pay a visit to Scotiabank Place (7:30 p.m., Sportsnet East, Team 1200). The Senators close out their pre-Christmas schedule Friday night in Raleigh, N.C., against the Hurricanes (7 p.m., Sportsnet East, Team 1200).

Around the boards

With Regin back on the shelf, MacLean said he'll dress seven defencemen against the Panthers tonight. The concussion suffered by Jesse Winchester in Buffalo on Tuesday also means that winger Bobby Butler will draw back into the lineup ... Craig Anderson makes the start in goal for Ottawa ... Senators prospect Shane Prince was released from Team USA's world junior camp earlier today. The cutting of the Ottawa 67's centre, a second-round pick by the Senators in the 2011 NHL Entry Draft, leaves the organization with four representatives at the 2012 world juniors, which begin Dec. 26 in Edmonton and Calgary. Forward Mark Stone (sixth round, 178th overall, 2010) will suit up for Team Canada, while Sweden's lineup includes centre Mika Zibanejad (first round, sixth overall, 2011) and blueliner Fredrik Claesson (fifth round, 126th overall, 2011). On the Czech roster is forward Jakub Culek (third round, 76th overall, 2010) ... Five Senators are on the ballot in fan voting for the 2012 Tim Hortons NHL All-Star Game, set for Jan. 29 at Scotiabank Place. To support Michalek, Daniel Alfredsson, Jason Spezza, Erik Karlsson and Sergei Gonchar, log on to vote.nhl.com or text a player's last name to 81812. Fan balloting closes on Jan. 4.