Tadić has found the back of the net within minutes of coming off the bench against Watford and West Bromwich Albion in the last week for Saints.
He also repeated the feat at Old Trafford last season, coming off the bench to score the winning goal and seal Saints’ first away league victory against Manchester United in 27 years.
Ahead of Saturday’s trip to Manchester, Saints boss Koeman has urged the Serbian to continue his hard work as he looks to break back into the starting line-up.
“Sometimes from the beginning it's not so important or not really that level what he showed when he came off the bench.
“It’s difficult because in the last part of the game you maybe get more space.
“I think we need Dušan on his level, and of course if he doesn’t start normally it’s the reason that I think another player in that position can bring more to the team.
“He has to keep working, keep fighting and show his qualities.”
While Koeman admits it’s difficult to change a winning side, Saturday’s game may present Tadić with the opportunity to come back in from the start.
“It’s sometimes difficult because if the team is doing well, the team is winning and the player has his impact when he comes off the bench, then you don’t change a winning team,” Koeman added.
“It’s a little about the system, a little about the opponent and in the aspects we change sometimes players or positions. We will see what happens on Saturday.”