Everton forked out £13.5million to land Niasse on a four-and-a-half year deal from Lokomotiv Moscow in February.
The Senegal striker managed just 152 minutes of first-team football last season and has been frozen out completely by boss Ronald Koeman after he replaced former manager Roberto Martinez.
Koeman made it clear to Niasse he wasn’t in his plans during the summer window but the 26-year-old opted to remain at Goodison Park.
Niasse faces a huge battle to force his way into Koeman’s thoughts but he did record a 21-minute hat-trick for the Under-23s in their 6-3 win at Reading, and boss Unsworth was quick to praise the forward.
“We were disappointed with the two early goals we conceded, but it woke us up,” Unsworth told evertonfc.com. “From then, I have to give massive praise to Oumar Niasse, who got us back in the game with a great header and followed it up with two more.”
He added: “Oumar has been great with this group of players – both in training and in games.
“His work-rate and the quality he showed to take his goals were fantastic. He got us back into the game at a time when we were second best.
“He’s been a great professional and he needs to carry on what he’s been doing.”
Southampton have announced plans for a two-week tour of China as part of their pre-season preparations for the 2018-19 season.