According to Dutch newspaper De Telegraaf (via Sport Witness), United scouts were in the stands to watch Janssen in Saturday's 4-2 home defeat to PSV Eindhoven in the Eredivisie along with officials from Paris-Saint Germain and Everton.
Janssen, 21, was not on the scoresheet against PSV, but he has been the Dutch top flight's most prolific scorer this season, netting 20 goals in 29 appearances, per WhoScored.com.
United need to be more effective in front of goal if they are to become Premier League title challengers again.
Anthony Martial's Sunday goal against Everton made him the club's top Premier League scorer this season, but he remains only in single figures, per Squawka:
Netherlands international Janssen could well be the answer for United, as he is a strong finisher.
However, the Old Trafford club must be wary of paying big money for another young player who's proved himself only in the Eredivisie, having yet to see a return on their £31 million investment in Memphis Depay last summer.
Another youngster in whom United are reportedly interested is Benfica's 18-year-old midfield star Renato Sanches.
According to Portuguese outlet A Bola (via the Mirror), United have told Benfica they are prepared to pay £40 million "up front" in order to sign Sanches in the summer.
Sanches has emerged as a significant force this season and impressed as Benfica have advanced to the quarter-finals of the UEFA Champions League.
He has shown great maturity beyond his years and is a true box-to-box midfielder capable of contributing in both attack and defence. See some of his best moments this season below:
United should again be wary of paying huge money for such a young player, but Sanches does look like he has the ability and strength to be a world-class central midfielder for years to come.
Elsewhere, agent Wagner Ribeiro has revealed Lucas Moura moved to PSG in 2013 despite being offered better terms by United because he wanted to live in the French capital.
Ribeiro was discussing the potential of the Ligue 1 giants signing Barcelona's Neymar in the summer.
He revealed that fellow Brazilian Moura could play a key role having joined PSG over United, per L’Equipe (via Metro's Max Miller): "He’s (Neymar) already played at Parc des Princes, Stade de France too. He loves Paris, it’s a beautiful city. And he knows a good guide, in Lucas. At the time, Lucas had a superior proposal from Manchester United. PSG was less strong, but the city made the difference."