If you stand still, you fall behind. It is one of the truest maxims in professional sport and why cycles of success are fluid, moving from one team to another. Such change can often happen over a period of years rather than months, but in the Premier League right now, the unique challenges facing the top two -- Liverpool and Manchester City -- have opened the door for the chasing pack to catch up.

Chelsea and Manchester United, in particular, looked to be in the first stages of a long-term rebuild prior to the coronavirus shutdown in March. Progress was being made at both Stamford Bridge and Old Trafford yet, realistically, the two most successful clubs of the Premier League era still appeared at least a season away from challenging for the title again.

Things have changed quickly. Man City's ongoing battle with UEFA over their two-year Champions League ban -- their appeal is being heard at the Court of Arbitration for Sport (CAS) -- has placed question marks over their ability to keep Pep Guardiola's squad together, never mind add to it, in the coming months.

Meanwhile Liverpool, who will clinch their first league title in 30 years soon after the Premier League resumes next week, are emerging from the shutdown with manager Jurgen Klopp admitting that the club will have to accept a new financial landscape that has left them unable, unwilling, or both, to seal a £53 million deal for RB Leipzig forward Timo Werner.

Werner, of course, is instead on his way to Chelsea, who are set to make the Germany forward their second new addition having already agreed a £33.6m deal for Ajax winger Hakim Ziyech to move this summer. Liverpool seemingly held all the aces -- Klopp, a side on the verge of the title, and a sense that Anfield is once again the place to be -- but the cost of the deal proved too much for them.

"All clubs are losing money," Klopp told Sky Germany. "How do I discuss with the players about things like salary waivers and on the other hand buy a player for £50m or £60m? If you want to take it seriously and run a normal business, you depend on income, and we have no idea how much the club will earn, especially because we don't know when we can start playing in front of spectators again."

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Liverpool will add to their squad at some point -- they simply must, having spent only £7.25m on new players (Takumi Minamino) since summer 2018 -- but their inability to sign Werner makes it clear that they will not be shopping at the top end of the market ahead of a season that could see them lose Sadio Mane and Mohamed Salah for a month at least when the Africa Cup of Nations comes around in January. And even if City overturn their two-year ban at CAS, they are unlikely to learn their fate until next month at the earliest. By then, the best deals may have already been done by clubs who have a clearer picture of their immediate future.

Liverpool and City, for different reasons, are having to stand still this summer, and both Chelsea and United are ready to take advantage.

Chelsea banked an initial £88.5m by selling Eden Hazard to Real Madrid last summer, and the fee could ultimately climb to £130m based on clauses in the deal. Having been under a worldwide transfer ban last year, that money remained unspent until now, with Ziyech and Werner accounting for the Hazard proceeds.

Chelsea have more to spend and, if he is smart, manager Frank Lampard can make his team title contenders next season if he is able to find the right additions in defence to bolster a squad blessed with emerging talent, exciting new signings and proven performers such as N'Golo Kante and Cesar Azpilicueta.

It is a similar story at United, where Ole Gunnar Solskjaer has emerged from a difficult winter with a team that now looks perhaps just two or three new players away from being contenders again. Unlike Liverpool, United have money to spend this summer and have deep pockets. Despite announcing last month that the club's net debt now stands at £429.1m, they are also operating with cash reserves of £90.3m, so United are well placed to take advantage of a market that will inevitably see transfer fees plummet in the wake of the coronavirus crisis.

Sources have told ESPN that United are front-runners to sign Borussia Dortmund forward Jadon Sancho and are also leading the pack in the race for 16-year-old Birmingham City midfielder Jude Bellingham.

The £70m January signing of Bruno Fernandes from Sporting Lisbon helped transform United's fortunes on the pitch prior to the shutdown in mid-March and, with Paul Pogba increasingly unlikely to get a move away this summer and Ajax's Donny van de Beek another possible arrival, Solskjaer could have formidable midfield options at his disposal next season.

In any ordinary summer, the champions would be expected to strengthen significantly to stay in front, but it is unlikely to happen for Liverpool this time. City's battle with UEFA means that their focus is not fixed firmly on recruitment. Chelsea and United are the two clubs most likely to take advantage. If they get it right, we can look forward to a four-way battle for the title next season.