The duo were a prolific and highly potent front pairing for the Lions – banging in the goals which took the SE16 outfit to two successive League One play-off finals.
And Gregory has been top of the club’s scoring charts for their last two seasons in the Championship.
But now he is set to move on when his contract expires at the end of next month.
Millwall manager Neil Harris told the South London Press recently that they had never been able to open talks over a new deal for Gregory, with attempts to broker negotiations never encouraged.
“He has been the spearhead of our attack for the last four years,” said club captain Morison. “Unfortunately he’s not going to be there next year. But it also shows he has got a pair of balls on him, because he could easily have just signed a contract and just settled.
“But he wants to try and better himself and decided to run his contract down. To be out of contract now, I have no doubt he’ll have a few suitors and he can pick where he wants to go.
“That’s the big decision he made, considering he is 31 in August. He took a massive gamble on his future and it seems like it has paid off.
“He’s asked my advice on things at times. What would you do? What do you think? My answer every time is that you have got to do what is best for you. That’s what everyone has to do in ever walk of life.
“The only tough situation was that he was risking an injury, which would have meant he wouldn’t have had a contract next year. But he has played on the edge still and been great. That shows his character.”
The barb aimed at Gregory by some of the Millwall fanbase is that he misses some big chances.
“If I was going to give one criticism of Gregs it is that he doesn’t score enough goals,” said Morison. “Sometimes he is clinical and sometimes not.
“That’s the difference if you look at a Billy Sharp or Teemu Pukki. They get one chance and they score it. But, on the flipside, the better teams you play for then the more chances you get. Maybe they get five or six chances and we don’t see that in the highlights reel, you only see the one or two they put away.
“You go to better teams then you are going to get more chances, you are going to score more goals. I wouldn’t be surprised if Gregs got 20 goals in the Championship if he played week in and week out and for a team which played well and created chances.
“Ultimately this season he has been in a struggling team. So to score 13 goals – and some big goals in there – is a decent record. I think it is the other side of his game, which is why whoever takes him, takes him.
“His general play – his willingness to work – is great. He is definitely not big time, he hasn’t got a chip on his shoulder and he’ll work really hard even if he is not scoring. He is always contributing, which is a massive part of the game when you play up front.”
Millwall need to replace Gregory. No easy task. He cost £250,000 from Halifax in 2014 but strikers – particularly proven in the Championship – do not come cheap.
“It is going to be very tough to replace him,” said Morison. “We have got Tom Bradshaw, who came in and unfortunately got injured. He’s got a decent scoring record in League One and hopefully he can fulfill his price tag and what the manager saw in him when they signed him at the start of the season.”
Morison has undoubtedly played a part in Gregory’s stock rising. The pair enjoyed major success in a 4-4-2 formation.
And the elder statesmen says that chemistry came naturally.
“There’s no formula to it,” he said. “I came back to the football club and one of my remits was to play in a front two with Lee Gregory and get the best out of him.
“I’m now labelled a targetman, which is fine, but I never have been a targetman here. I wasn’t the first time around. I’ve adapted my game to get the best out of Gregs and Gregs has adapted his game to get the best out of himself.
“That has been shown in the last four years before this one, definitely the first three years.
“When something works, it just works.
“We didn’t overly work on being a pair in training, we might do some bits but normally the finishing drills is a lot of individual stuff.
“It just works – look at his goal on Tuesday night [against Bristol City]. I know where I’m heading it and he’s there. He’s always half a yard ahead of someone else. He knows where the ball is going.
“He’s right up there [as a striker partner Morison has played with]. Goals speak for themselves. When I first came here I played with the gaffer [Neil Harris]. He ended up with 18 goals and I got 23 – we got promoted to the Championship and he left.
“I had a range of partners after that. I’ve been away and played with Holty [Grant Holt] at Norwich. To be fair, I spent a lot of time on my own up front there.
“Gregs is right up there. Apart from my first season at Millwall with the gaffer it was the only other time I’d really played in a two with another striker.
“It has been great playing with him. Wherever he ends up in the summer then I’m sure he’ll do very well.”