Lopez, who was only recalled from a loan spell at League Two Barnet earlier this week due to injuries at Stevenage, was the class act as he scored once in the first half and twice after the break, with Lucas Akins adding the fourth.
The Spanish striker, who began his career with Atletico Madrid playing in the same youth team as Fernando Torres, scored in the 41st, 50th and 64th minutes as his side won for only the second time in 10 games.
Lopez's opener, the first goal Sheffield United have conceded in seven games, came when he turned Neill Collins inside the box and finished coolly past George Long.
Lopez then attempted an audacious curled effort from 20 yards which Long did well to tip over.
Gavin Mahon was the architect of Stevenage's second goal, playing a ball through a static Blades defence for Lopez to finish.
United pushed to get back on level terms as Callum McFadzean and Harry Maguire both went close with headers.
But it was Lopez who was next to find the net, scoring against the run of play as he fired in from the edge of the penalty area.
And it got even better for Boro just four minutes later as Lucas Akins added a fourth with an individual run and shot.