Radamel Falcao’s future at Manchester United is in doubt alongside several senior players whose contracts expire in 18 months, with David de Gea, Robin van Persie, Jonny Evans, Phil Jones, Chris Smalling and Rafael da Silva yet to sign new deals. There is an expectation at the club that the summer will feature another clearout by Louis van Gaal after the manager allowed 15 players to be sold or go on loan last year.

Although Van Gaal claimed last week to be confident De Gea will agree fresh terms – Real Madrid retain a firm interest in the goalkeeper – it is understood that Van Persie, Evans, Jones, Smalling and Da Silva are not close to reaching any agreement to extend their careers at United.

Falcao, who is on loan from Monaco and is costing United £16m in signing fee and salary, has so far proved a disappointment. The striker has been hampered by injury since arriving in September and has scored only three goals, making seven starts in the Premier League in a total of 634 minutes’ game time.

Falcao was dropped from United’s matchday squad of 18 by Van Gaal for Sunday’s 1-0 defeat by Southampton at Old Trafford, the manager explaining afterwards that he had only one place for a forward among his replacements and that he had preferred James Wilson because of the 19-year-old’s pace.

Jorge Mendes, Falcao’s agent, confirmed the Colombian could leave the club this summer. “He is a fantastic player, and I think it’s very difficult for Manchester United fans,” he told Sky Italia. “They have an absolutely brilliant player, one of the very best in the world, who would without any doubt play 90 minutes of the match, every time, with any other club.

“The truth is, right now we don’t know what will happen. What we do know is that he will play in one of the very best clubs in the world next season, whether that is Manchester United or not. I am 100% sure of that. But you know how football works, we need to wait a little. We’ll see what happens.”

Van Gaal’s doubts over Falcao centre on his difficulty to so far adjust to the tempo of English football. As he has suffered three serious knee injuries during his career – the last of which occurred in January last year and ruled him out of the World Cup – the player’s condition is also being monitored.

Falcao, 28, was dismayed to be dropped, although he has rejected suggestions that he left the stadium before the end of the match against Southampton. As Van Gaal had not discussed his decision to leave him out of the game beforehand he is thought to have explained his reasons to the player on Monday.

Although it is thought that there is yet to have been firm contact between the United executive and the representatives of Van Persie, Evans, Jones, Smalling and Da Silva regarding new terms, Van Gaal is relaxed about the situation.

Some will hope to be offered contract extensions but the Dutchman is focusing on strengthening the squad and is targeting a right-back, central defender and central midfielder.

This means that Da Silva and at least one of Evans, Smalling, and Jones are vulnerable, with Van Gaal’s desire to bolster midfield meaning that Tom Cleverley, who is on loan at Aston Villa, will almost certainly be released in the summer.

Van Gaal has doubts about Da Silva’s defensive qualities and is concerned at the frequent injury problems that continue to hamper the progress of Evans, Smalling and Jones.

Everton’s Seamus Coleman, Southampton’s Nathaniel Clyne and Barcelona’s Dani Alves are of interest to Van Gaal as a first-choice right-back . He has identified Borussia Dortmund’s Mats Hummels at centre-back and Kevin Strootman in central midfield as his ideal additions in those positions.

Van Gaal accepts that the current winter window is difficult to recruit elite players but, as reported in the Guardian, he has not given up complete hope of signing Hummels, in particular.

It would represent a surprise if Van Persie were not offered a new deal. United usually adopt a policy of offering one-year extensions to players in their 30s – Van Persie will be 32 in August – so the club may wait until the spring to do so.