Head coach Robbie Neilson described Hearts' loss at Ibrox as "a reality check" as he prepares to rebuild his squad following promotion.
The champions suffered their first away defeat of the Championship season, losing 2-1 to 10-man Rangers.
"We've had a great season, but, when we go up, it's going to be a whole new ball game," said Neilson.
"There will be more games like this against good teams and we'll need to work hard over the summer."
Neilson revealed prior to kick off that full-back Adam Eckersley will not be offered a new contract, while Jason Holt, Brad McKay and Scott Robinson will also move on at the end of the campaign.
"It's important that we change the squad a bit and get better players in," he told BBC Radio Scotland.
"We're a Championship team that's been promoted, so we must make sure we bring quality in.
"I'll be looking at four, five, maybe six players.
"I've got the nucleus of a really good save and I want to keep hold of the ones I want."
On-loan striker Genero Zeefuik scored a late consolation goal at Ibrox as Hearts failed to take advantage of a red card for Rangers skipper Lee McCulloch moments before half-time.
By the time of the dismissal, the visitors were trailing to goals from Kenny Miller and Haris Vuckic.
"I don't think we did enough to win the game," added Neilson.
"First half, we didn't do enough of the things we were trying to work on. We let them press us and it wasn't until we went two up top that started to cause them some problems.
"Second half, we dominated, but when teams sit in like that, it's really hard to break them down.
"At the end of the day, we're champions and we've had a fantastic season.
"It can be difficult for the players to really get motivated and Rangers still have a lot to play for.
"The intensity level wasn't quite there for us."