Forward Romario Sabajo to miss out for Morton
Morton will be without Romario Sabajo when St Johnstone visit after the Dutch attacker was taken to hospital following Saturday's win over Alloa with an ankle problem.
Ricki Lamie will be absent due to a hamstring injury with the Greenock side also without the firepower of Peter MacDonald, who has a knee injury, and Alex Samuel who suffered a concussion recently.
Manager Jim Duffy said: "It's good to go into this game with a win behind us after a disappointing run of results.
"We will need to play at our very best and hope that an excellent St Johnstone side are off their game if we are to get through."
Meanwhile, Tommy Wright urged his St Johnstone side to guard against a Scottish League Cup shock at Cappielow Park.
The Scottish Championship club set up the quarter-final tie against Saints with a 3-2 home win over top-flight side Motherwell.
And the McDiarmid Park boss knows that after bringing the Scottish Cup to Perth for the first time in the club's history in 2014, the holders were knocked out the following season by another second-tier side, Queen of the South, in Dumfries.
And while St Johnstone beat Morton 1-0 in a League Cup tie two seasons ago, the Northern Irishman is wary of another trip to Greenock.
"The lesson for us will be that we can't take it for granted because they have already beaten Premiership opposition this year," said Wright, who will be without suspended skipper Dave MacKay after he picked up a red card in Saturday's 1-0 Ladbrokes Premiership win at Inverness.
"And we went out of the Scottish Cup last year to a team from a league below us.
"It is cup football, it is about who turns up on the night.
"Cup football is littered with big teams going out to teams from lower leagues, in all levels of football in every country.
"It wasn't a great game two seasons ago and we got a goal in the last minute.
"They had come off the back of beating Celtic. This is similar circumstances. They have beaten a Premiership club.
"They will be keyed up for it and we are expecting a tough game.
"You have to earn the right to win football matches and you have to respect the opposition.
"All you need is three or four players to go into it underestimating (your opponents), not being focused and you can get beat."
Wright is placing no real significance in the convincing 3-1 win over Championship leaders Rangers at Ibrox in the previous round.
Saints are the only team to beat Mark Warburton's side this season but the St Johnstone boss said: "Beating Rangers is gone for us. Our focus is on this game.
"Winning against Rangers got us a bit more profile and the manner of the performance was convincing but it has no effect on Tuesday night's game.
"Generally that win should give the players a lift but the performances since then have been pretty good generally.
"We are preparing like we do for every opposition - thoroughly - and we will be ready for the game."
Source: PAR
