Diego Costa has previously admitted that he actively looked for a way out of Chelsea last summer.
And in January the Spain international was subject to heavy interest from the Chinese Super League, with Tianjin Quanjian willing to pay Costa £30million-a-year.
Meanwhile, Zlatan Ibrahimovic is yet to commit another year to Manchester United with his contract ending in the summer.
Frank Lampard believes Chelsea may find it hard to replace Diego Costa if he leaves
Lampard suggested Zlatan Ibrahimovic doesn't do as much as Costa does for his team
Both strikers have scored 17 goals in the Premier League this season and been pivotal in their respective side's success.
But Lampard believes Chelsea would find it harder to replace Costa than United would to find a stand-in for Ibrahimovic.
"There are players out there who are that sort of target man/ goalscorer who can run the channels a little bit," Lampard told Sky Sports.
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"When you're playing up there on your own you need to be a bit of an all-rounder.
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"That's where we talk about Zlatan Ibrahimovic being brilliant, but occasionally people say what does he do for the team?
"With Costa, his mobility and presence and his quality is the difference so again to replace that for Chelsea is a big ask."