The Wales striker gave any Premier League clubs looking for a goalscorer before the transfer window shuts another nudge with his 17th-minute strike.
The Lions are desperate to hold onto their top scorer, who has been linked with Fulham and Wolves, and hope to tie him to a new contract.
But boss Kenny Jackett may be tempted to switch his phone off between now and Monday evening just to be on the safe side.
James Henry added a second before half-time as Millwall moved up to eighth, just three points shy of the top six.
It all added up to a second wasted trip to south London in the space of a month for the 270 hardy Barnsley fans after the original fixture was called off a couple of hours before kick-off.
Their team nearly gave them something to cheer in the 15th minute but Hugo Colace fired over from 12 yards.
However, Millwall dominated the first half and went ahead two minutes later when Henry's low cross found Morison all alone at the far post to tap the ball home.
Barnsley briefly threatened again when Colace was put through but his chip was comfortably dealt with by home keeper David Forde.
And the Lions went 2-0 up in the 37th minute when Henry's 20-yard drive took a huge deflection off Tykes full-back Matthew Hill, wrong-footing Luke Steele as it flew into the net.
Morison should have made it three when Liam Trotter sent him through one-on-one with Steele, but his first touch let him down and the Tykes keeper was able to block his shot.
Morison then charged down a Steele clearance but Trotter was unable to take advantage, and Alan Dunne crashed a shot over before the interval.
Barnsley livened up after the break and Danny Haynes, who scored twice on his debut in Tuesday night's win at Doncaster, fired a decent chance wide.
At the other end Morison was still making a nuisance of himself and had a shot bravely blocked by Jason Shackell before teeing up Henry who could only divert the ball over with his knee.
Jacob Butterfield could have dragged Barnsley back into the game with 10 minutes remaining, but their chances disappeared with the midfielder's shot as it flew wide.