Puncheon, who has joined the Lions from Southampton until January 3, scored on his debut in a 1-0 win at Middlesbrough on Saturday.
However, Jackett claims the 24-year-old has plenty left to prove before the Lions can even consider making a move to sign him on a full-time basis.
"It is a great way to start your career at a new club and he is a player with a lot of ability," Jackett told Sky Sports' website.
"He has his Millwall career in front of him now and we will see how he responds from an impressive debut.
"It is a loan deal at the moment and that is all it is.
"He played very well on his debut, but that was only one game and it is only the early stages of his loan, so we will have to see how things progress with him first before considering turning it into something more permanent."
Millwall are short on numbers in attack following injuries to key players such as Chris Hackett, Theo Robinson, Kevin Lisbie, Neil Harris and Daniel Schofield, but Jackett is hoping to see three of those stricken stars fit for selection this weekend when the Lions travel to Preston.
He said: "We are hopeful of having the likes of Kevin Lisbie, Danny Schofield and Chris Hackett back for the weekend, but it is touch and go, so we will have to assess their situation closer to the time."