"To come to the Premier League (in January) and perform is not easy," Moyes told British media.
"What goes amiss is that he's a real hard worker, covers the ground, gives us some other things as well.
"He brings a little bit of flair, good on the ball, a bit of natural talent. What we've needed from him — when (Manuel Lanzini) was injured in January — was to give us a goal, or an assist, and today he came up with the goods."
Asked if there was a possibility of keeping Mario's beyond the current campaign, Moyes said: "I wouldn't want to even comment... because I won't make any decisions or even look at it (yet). I'll take my time and see what we've got."
Moyes also lauded fullback Arthur Masuaku, who made his first appearance after serving a six-game suspension for spitting at an opponent during the FA Cup loss to Wigan Athletic in January.
"We've missed his natural balance; whether you see him as a left back, left wing back, left winger, he's a little bit of all of them at different times in the game," the manager added.
"In a way, I think he has benefited from his time off, because we've used that time to practice his finishing, his crossing and he's positionally a little bit better... although it was six games it felt like six months we were missing him."
The win over Southampton lifted the Hammers to 14th place, five points above the relegation zone with seven matches remaining.
(Reporting by Hardik Vyas in Bengaluru; editing by Amlan Chakraborty)
