Alexander Romanov will miss five to six months and have right shoulder surgery after Mikko Rantanen’s hit last Tuesday, the Islanders announced.

That was expected after Romanov went down in a heap following Rantanen’s hit that took him out of Tuesday night’s win over Dallas.

The unexpected injury news came when the Islanders were also without Jean-Gabriel Pageau — with No. 44 being declared week to week with an upper-body injury.

That forced Cal Ritchie to move up to the third line, with Casey Cizikas centering the fourth, as the Islanders rejiggered their forward group to compensate.

Anders Lee skated on the top line with Mat Barzal and Jonathan Drouin; Simon Holmstrom and Max Shabanov were on either side of Ritchie; Max Tsyplakov rejoined the lineup to skate on a fourth line with Cizikas and Anthony Duclair.

The second line of Emil Heineman, Bo Horvat and Kyle Palmieri was the only one unaffected.

“When I saw [Romanov] going down, I really thought he had a lot more serious injury,” coach Patrick Roy said. “It’s a serious one, because his season is over, but I thought maybe a broken neck or something like this.”

Asked for clarification on whether Romanov is out for the entire season, including the playoffs, an Islanders spokesman reiterated that the timeline is five to six months.

That would set a possible return date in April or May — during a potential playoff run — which makes the Islanders one of the league’s first potential test cases for the rule changes that stipulate the 20-man roster that dresses for a playoff game must be cap compliant.

So if Romanov were to go on LTIR — he currently is not, but that is a possibility — and the Islanders add salary at the deadline, it could put them in a bind over how to stay compliant in the postseason, should Romanov eventually return.

Though Rantanen was not suspended for the hit on Romanov, which incurred a game misconduct, he was suspended Sunday for one game following a second illegal hit — this one on Calgary’s Matt Coronato.

That suspension automatically was triggered by Rule 23.6, which states that any player who incurs two game misconduct penalties before playing in 41 consecutive games without such a penalty will be automatically suspended.

The Islanders already had filled Romanov’s roster spot by calling up Marshall Warren, and it’s unlikely there will be more roster moves in the near future.

Adam Boqvist stayed in Sunday’s lineup opposite Scott Mayfield on the third pair, with Warren a healthy scratch.

Kraken coach Lane Lambert spoke with New York media pregame for the first time since the Islanders fired him in January 2024.

Lambert described himself as “surprised” when the Islanders let him go and hired Roy, but he did not delve further into the subject.

Asked if there were anything he would do differently with the Islanders, he said the matter was “past history.”