But he slotted home Leeds' second goal from Sanchez Watt's low cross, and then blasted a left-footed effort past Millwall keeper David Forde after the hosts broke from deep three minutes into injury time.
Simon Grayson said: 'We were unsure whether he could do it at this level, but we knew he was a goal-scorer. He's got fantastic feet, he's very composed and he takes chances when he gets them.
'We knew we had a player that could finish. He's got an eye for a goal and I'm delighted for him.'
It was a startling introduction from a man who was released by San Jose Earthquakes in May last year and joined Leeds 12 months ago after buying a one-way ticket to England.
In the 23rd minute, Howson teed up Luciano Becchio for a shooting chance from 10 yards but the Argentinian could only blaze wildly over the crossbar.
Three minutes later Howson burst into the box and showed some delightful footwork to engineer the space to rifle a low shot against the outside of a post as Millwall survived.
The Lions' goal began to lead a charmed existence and after Sam had a shot cleared off the line in he 27th minute, he drew Leeds level five minutes later.
Watt drifted across the face of Millwall's penalty box before finding Becchio. He kept the move flowing with a neat touch into the path of Lloyd Sam, who struck a fierce low right-footed shot past Forde and into the bottom right-hand corner.
Somma spent the end of last season on loan at Lincoln City as he tried to come to terms with English football.
But he has now proved his worth to Grayson, who will surely find it hard to deny him a more involved role in United's campaign.
However, this was more than just a one man show. Leeds' Championship rivals will look at this performance with alarm, because on this evidence football's sleeping giant is starting to stir.
Leeds finished each half exuding confidence and menace in equal measure, and the Elland Road faithful loved it.
Watt was outstanding, Sam a constant threat and Becchio suggested he is more than capable of handling the step up to football's second tier.
Millwall boss Kenny Jackett said: "It was like a Cup final for Leeds. They created a lot of chances, and while at one stage I thought we were going to weather the storm, they were sharper than us. They were on the front foot more than we were."