With no more room for error in the title race, the visit of the Tannadice side will be crucial in terms of keeping the destination of the SPL trophy in Rangers' own hands so there is already plenty at stake.

But Smith's players will also be keenly aware that the match at Ibrox provides the final opportunity to impress the manager before he selects the team which will face Zenit St Petersburg in Manchester on Wednesday.

"The form has to be good and everyone wants to play in the big games," said Whittaker.

"Your form going into that game is going to be important and the manager will see it how he sees it."

Whittaker can play in midfield and on either flank at full-back and hopes his versatility will continue to serve him well ahead of the clash against the Russians.

He added: "I've been switched about a bit and I'm just trying to do my best in whatever position that I'm playing.

"I'm quite versatile and I can adapt to different positions so I'm happy about that.

"I'm happy with my current form and the team is playing well. We are getting great results and we are having a good season so far.

"Just to be where we are at this stage of the season is fantastic and hopefully we can finish it off."

Whittaker does not believe the fear of injury will be at the forefront of the Rangers players' thoughts when they take on United ahead of arguably the biggest game of their careers.

"Hopefully that won't happen," he said.

"Anyone can get injured at any stage and it would just be bad luck if that was the case.

"Hopefully we come through it injury-free."

Rangers this week failed in their bid to have the match with United postponed until after the UEFA Cup final, with the Scottish Premier League refusing to sanction a further extension to the season.

"We just have to concentrate on the job at hand and not think about what might have been," Whittaker said.

"We need the three points and that's what we will be concentrating on when the game comes around."

A win against United would allow Rangers to slash Celtic's lead at the summit to a single point ahead of the champions' own game against Hibernian 24 hours later.

But, with Hibs, Dundee United, Motherwell and Aberdeen all vying for third place and a UEFA Cup berth for next season, Whittaker believes there is no need to offer encouragement to his old club-mates at Easter Road.

"Every team is pushing for that UEFA Cup spot and the Hibs boys are no different," he said.

"I'm sure they will be trying their best come Sunday."