Manchester United warned they will have to spend big to sign Ajax goalkeeper Jasper Cillessen

Manchester United will have to offer a significant sum if they are to secure the services of Netherlands international Jasper Cillessen this month, according to current Ajax manager Frank de Boer.

The Premier League title hopefuls have already completed the signing of one goalkeeper during summer transfer window to date, with Sergio Romero joining on a free transfer in July after leaving Sampdoria at the end of his contract in Serie A.

However, with first-choice David de Gea left out of the starting lineup for Saturday's Premier League curtain-raiser against Tottenham amid sustained interest from Real Madrid and former deputy Victor Valdes also set to leave the club after a public row with Louis van Gaal and Anders Lindegaard evidently surplus to requirements, United currently only have 22-year-old Sam Johnstone in reserve.

The Telegraph reported last month that an agreement to sign Cillessen in the event that De Gea does return to La Liga had already been agreed with Ajax and such a pursuit was further strengthened by the Dutch champions' disappointing 5-4 aggregate defeat to Rapid Vienna that means they will not play in the Champions League proper this season.

The 26-year-old shook off a calf problem to play in both legs of that tie, although Uefa rules stipulate that his involvement would not prevent him from playing in the competition with another team. United were recently drawn against Club Brugge in the draw for the play-off round and stand just two matches away from returning to Europe's elite club tournament after a one-year absence.

Despite their premature Champions League exit, however, De Boer insists that Ajax continue to fight against the possible exit of Cillessen and will only sanction his departure in the event that they receive an enormous fee.

"Man United will have to pay a huge price for Cillessen," he told The Mirror. "Let it be clear that we are going to take a really strong attitude and we will fight against him leaving at this stage. Why? Because we have seen all our three alternatives go out of sight and it gets harder and harder to find a decent replacement.

"To make it clear, Jasper is getting more expensive by the week, although there are certain figures that we can't turn down. This far in the window, I hope we hear nothing any more from Manchester United."

Ajax open their 2015/16 domestic campaign away to AZ Alkmaar later on 9 August, while United had a Kyle Walker own goal to thank for their opening day victory over Mauricio Pochettino's Tottenham at Old Trafford.

It was far from a vintage performance from the hosts, but new signings Matteo Darmian, Morgan Schneiderlin and Memphis Depay all showed early promise along with the likes of centre-back Chris Smalling, Ashley Young and Juan Mata.

Van Gaal confirmed before the match that De Gea simply was not focused enough to merit selection in light of the persistent speculation surrounding his future over recent months.

Although the manager later claimed it remains possible that he could yet play for the club again in future.