IRVING, Texas — The Giants should consider themselves officially on notice starting with Sunday’s showdown with the Jets.

Fresh off one of Big Blue’s biggest flops in a roller-coaster season, team co-owner Steve Tisch made it clear to The Post on Wednesday that ownership considers the final five games win-or-else time for the Giants.

Coach Tom Coughlin and general manager Jerry Reese didn’t exactly get a ringing endorsement from Tisch, either, after the Giants wasted the chance to take control of the woeful NFC East last Sunday with a 20-14 loss to the Redskins that dropped them into a first-place tie with Washington at 5-6.

“It’s been a very frustrating season,” Tisch said during a break in NFL owners meetings. “We’re still in it, for sure, but that’s more of a statement of how unique the NFC East is.”

Tisch’s frustration was magnified by the fact that the Giants came out so flat after their bye week against a sub-.500 team and just two weeks after nearly toppling the then-undefeated Patriots.

Tisch bemoaned the Giants’ inconsistency, which has seen them start 0-2, win the next three games, then alternate wins and losses for a month until last Sunday’s embarrassment at FedEx Field.

“Under Coach Coughlin, we’re fighters,” Tisch told The Post. “[But] sometimes we fight harder than the previous week. We’ve got to stay focused and go out there as a team.”

Tisch wants that consistently strong effort to start this Sunday at MetLife Stadium against the 6-5 Jets, then continue in a difficult final month that includes a visit by the 11-0 Panthers and a trip to the 8-3 Vikings.

“We have five games left, and all five games are important,” Tisch said. “There’s not a lot of margin for error.”

The Giants were among the NFL’s most injury-riddled teams the previous three seasons, and this year has been no different, with 11 players — many of them key starters — already on injured reserve.

But with Big Blue staring at the possibility of missing the playoffs for a fourth consecutive season since winning the Super Bowl in 2012, Tisch wasn’t in the mood for excuses Wednesday.

Tisch’s impatience appears to be heightened by the fact the Jets are next on the schedule.

Tisch said he considers it a strong rivalry — unlike several of his players in comments downplaying that angle Wednesday — and is so excited, he plans to make a bet (most likely for dinner) with Jets owner Woody Johnson about the outcome.

Asked if the final five games are especially important for the fate of Coughlin and Reese, Tisch didn’t offer any reassurances in his interview with The Post.

“The next five games are important for everybody,” Tisch said. “[The fate of] Jerry and Tom is not a discussion to have today, but I certainly don’t want to disappoint our fans as we get into the last month of Giants football.”

Even more ominous for the Giants’ brain trust was Tisch’s response when pressed on whether another season without a playoff trip would be acceptable to Tisch and co-owner John Mara.

“I would not be happy with our season ending on Jan. 3,” Tisch said.