A year on, can the 28-year-old be the man to lead Rangers to the group phase of Europe's premier club competition for the first time in more than a decade as they visit PSV Eindhoven in the second leg of their play-off with the scores tied at 2-2?

After four goals in his last five games, BBC Scotland examines Colak's start to life in Glasgow and looks at where his promising introduction leaves Alfredo Morelos, who will remain at home after being dropped by manager Giovanni van Bronckhorst over his attitude and fitness.

Four goals, seven starts & bags of 'high energy'

Despite rocking up to Ibrox with a 2-1 lead last August, there was an expectation that Malmo would be rolled over by Gerrard's Rangers in the second leg.

And, after Morelos' 18th-minute opener and Bonke Innocent's red card just before the break, it appeared that expectation would be realised.

But two ruthless Colak finishes in the space of four second-half minutes stunned Rangers, who couldn't find a way back into the tie.

Then on loan from PAOK, Colak's Ibrox strikes were two of 19 goals he netted during his season-long spell with the Swedish side, an impressive run of goalscoring form that prompted Rangers to make their move.

Following his transfer, for a reported £1.8m, Colak has scored four goals in seven appearances across all competitions.

It could have been five after having a header ruled out for a marginal offside call on his debut at Livingston.

The former Rijeka man, who averages almost six touches in the opposition box per 90 Scottish Premiership minutes, has shown a clinical edge by scoring two league goals from just three shots on target.

If Colak can keep up his current rate of scoring, he will be on his way to a healthy total come the end of the season, but the Croatian's off-the-ball work is also endearing himself to his manager.

Van Bronckhorst wasn't tempted to throw on Morelos in last week's first-leg draw with PSV, a game in which Colak scored, before going on to explain how his new forward gave his side "the energy they needed".

"His work-rate when we don't have the ball is really good," the Dutchman told BT Sport. Meanwhile, former Rangers striker Kenny Miller hailed Colak's performance as his best for the club so far.

That level of performance, if not more, will be required from the Croat in Eindhoven as the Ibrox side seek to reach the promised land of the Champions League groups.

What about Morelos?

A game has passed since last week's thrilling first leg and, while Colak didn't add to his goal tally in Saturday's 2-2 draw with Hibernian at Easter Road, the actions of his direct rival for the number nine spot have strengthened the Croatian's hand.

Given what Morelos has produced for Rangers over his five years at the club, it would be unfair to assume Colak will be Van Bronckhorst's main man for the foreseeable future.

However, after the weekend red card, the eighth of his Rangers career, Morelos' discipline, as well as his fitness, has been questioned again and has led to the Colombia striker being dropped from the squad travelling to Eindhoven.

Fitness and attitude concerns have been cited as reasons for his absence, issues that appeared to be ironed out, but those problems have now resurfaced after a frustrating start to the season for the 26-year-old, who missed the end of last term with a thigh injury.

"You need to be fit in training if you're going to be given the opportunity to get into a starting XI," former Ibrox striker Kris Boyd said on Sky Sports prior to Saturday's Easter Road encounter.

"Morelos needs to get his finger out because he's lacking severely in fitness. The goals don't move, but you've got to be fit to play games of football nowadays. You need to be fit in training. Looking at Morelos, he's not."

After a long-term injury and a lack of game time, an element of rustiness has to be expected from any player and Morelos is no different.

The issue now, however, is further minutes in the short-term are going to be limited for the Colombian, especially as he prepares to serve a two-game domestic suspension.

From the outside, it appears to be a bold and ruthless call from the Dutchman, who is leaving behind a man who has scored a record-breaking 29 European goals for Rangers.

Prior to the season kicking off, there was optimism about Morelos committing to a new contract, with his current deal having under a year left to run.

However, with just over a week remaining of the summer transfer window, reports suggest those talks are now on hold.

Perhaps the door isn't completely shut on Morelos. Perhaps there is a way back. But, with senior players believed to have backed Van Bronckhorst's decision to drop the striker, that road back could be a long and unforgiving trek.

'Colak could solidify his place' - analysis

Former Scotland midfielder Michael Stewart on BBC Sportscene

Morelos is going to be in a difficult position now. He's going to miss more games and Colak is going to get more time on the park. He could score more goals and solidify his position as the number nine.

I'm convinced Morelos would have thought he was going to start away at Hibs because he needs games. He's not started and that red card was pure frustration. That's what happened in the earlier part of his Rangers career.