Manchester United's owners, the Glazer family, have sanctioned a three-year plan to restore the club's on-field fortunes, with sources having told ESPN FC there is now an acceptance the team is unlikely to challenge for the Premier League until the 2021-22 season at the earliest.

With traditional rivals Manchester City and Liverpool setting the standard in domestic and European football, the United hierarchy have noted how at Anfield, in particular, they have arrived at their status after rebuilding behind the scenes and backing manager Jurgen Klopp during the German's difficult first 18 months in charge.

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United, the most successful club in English football with 20 league titles, have not won the Premier League since 2012-13 -- the final season of Sir Alex Ferguson's 27-year reign as manager.

And in the six seasons since, under the management of David Moyes, Louis van Gaal, Jose Mourinho and now Ole Gunnar Solskjaer, United have finished an average of 21.5 points behind the eventual champions.

Under Solskjaer, United have made an unconvincing start to the campaign, with the team sitting in eighth position, 10 points behind leaders Liverpool after six games.

Executive vice-chairman Ed Woodward said on Tuesday, during a conference call with the club's investors, that Solskjaer needed to be afforded "patience" in his attempts to overhaul the squad and make United competitive again at the top level of the Premier League.

And sources with knowledge of the club's plans have told ESPN FC that the "patience" of the United hierarchy extends to the belief that the so-called "cultural reset" cited by Woodward last season will require three full seasons, and accompanying transfer windows, before the mistakes of the post-Ferguson years can be properly rectified.

As well as investing in the playing squad, United are also determined to improve their structure off the pitch, with the search for a technical director still ongoing.

But while United view this summer's transfer dealings as being a positive signpost of the way ahead, it is only regarded as a starting point, with the gap between Solskjaer's squad and those at Liverpool and City still vast in terms of depth and quality.

And although the three-year plan is not a guarantee of Solskjaer being at the helm for the entire period of the rebuild, the Norwegian is aware of the club's decision to build for the long-term rather attempt a quick fix.