Manchester United plan to follow up their £150.1m summer transfer spend with further strengthening in the January window with the club buoyed by the acquisitions of Colombian striker Radamel Falcao and Argentinian winger Ángel di María despite not having Champions League football this season.

Although the 20-times champions paid £59.7m for Di María, a £6m loan fee for Falcao, £27m for Luke Shaw, £28m for Ander Herrera, £16m for Marcos Rojo and £14m for Daley Blind, there remains a sizeable sum at Louis van Gaal’s disposal and the 63-year-old is free to invest this as he wishes.

Van Gaal is pleased with a window’s work that has seen the arrival of these six headline players, according to high ranking sources at Old Trafford. The United executive was clear that the attraction of playing under the Dutchman, plus the draw of the club’s world-famous name were the determining factors in being able to attract footballers of the elite status of Falcao and Di María.

However, the manager still wants to add a top-line central midfielder and central defender to his squad. In the winter window he hopes to buy Kevin Strootman if the Roma midfielder recovers from a serious knee injury that has ruled him out since March.

The 24-year-old played under Van Gaal when he was in charge of Holland and is minded to make the move to United provided the two clubs can agree a deal that may be priced at £30m.

Strootman is expected to return to action in the autumn. Speaking to the Guardian in June, his agent, Chiel Dekker, said: “He is of interest to Louis van Gaal but not related to Manchester United at this point.”

After deciding against moving for Juventus’s Arturo Vidal Strootman in the summer, Strootman is the type of dynamic, all-action midfielder Van Gaal wants for his United team.

Asked what kind of player Strootman is, Dekker said: “He’s all - so he’s creative, he’s box-to-box.”

Strootman, who made 24 Serie A starts and scored five goals in the league before suffering the injury against Napoli, played for PSV Eindhoven before moving to Roma for about €20m. Having previously played for Utrecht and Sparta Rotterdam, Strootman has 25 Holland caps and three goals.

Van Gaal will again consider moving for Mats Hummels, though there is a recognition that any transfer for the Borussia Dortmund central defender may be more likely to occur next summer rather than in mid-season. The 25-year-old had been a target for the window that has just closed but after losing Robert Lewandowski to Bayern Munich in the close season, head coach Jürgen Klopp was not prepared to lose another of his major players. With Hummels also the Dortmund captain it is also understood Klopp would not have signed a new contract if he had been sold. However the stance could change with Van Gaal monitoring Hummels’s availability.

Wayne Rooney hailed Falcao’s purchase and admitted he did not know if the No9’s addition might change his own position as a striker for Van Gaal: “I obviously don’t know how Falcao’s arrival could affect my role – that’s something you’ll have to ask Louis van Gaal. I think it is a fantastic signing. He is one of the best in the world. It is a real show of intent by Manchester United.

“It is a thrill. We have an exciting team. I am looking forward to the challenges ahead and I want it to be successful.”

So far this season Rooney has scored just once in three United games. It is thought that he may be deployed in a more withdrawn role to accommodate Falcao and Robin van Persie as the forwards in the side.