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The Bayer Leverkusen starlet has emerged as one of the most sought-after talents in European football.
Sides from across the continent are monitoring both his development and situation in Germany.
Premier League suitors appear to be heading the chase for his signature, with it also suggested that Liverpool boss Jurgen Klopp may be tempted to join the clamour for a fellow countryman.
Chelsea, though, have already shown that they can fend off the Reds when it comes to pursuing top targets, with a £54 million (€60m/$68m) agreement in place for RB Leipzig striker Timo Werner.
Frank Lampard has also tied up a swoop for Hakim Ziyech and has Leicester left-back Ben Chilwell in his sights.
Similar ambition is expected to be shown at Old Trafford, with United in the market for the likes of Jadon Sancho and Jack Grealish, but Hamann believes Stamford Bridge is showing itself to be the best fit for Havertz.
The former Germany international, who spent time in England with Newcastle, Liverpool and Manchester City in his playing days,has told Sky Deutschland: “Chelsea and Manchester United are fighting for fourth place in the Premier League.
“If United miss out on the Champions League again, the Red Devils are at risk of falling behind the other clubs.
“United has lost its appeal in recent years. They have been the measure of things for many years, they are still the largest club in England, but they have major internal problems.
“If there was a player on the market in the past, Manchester United was the first point of contact for years. I believe that is no longer the case today.
“When a player like Kai Havertz changes clubs and signs somewhere for five years, the long-term perspective is more important than participating in the Champions League next season.
“The player may have to make the decision in the next few weeks and cannot wait until the season in England, which runs until the end of July or beginning of August, is over. Perspective is important for a player, but Manchester has been in a vacuum for years.
“On the other hand, if I look at Chelsea, they have a coach with Frank Lampard who did a great job for the club as a player,and I have the feeling that the Blues are growing again.
“They had the two-year transfer embargo and, with Mason Mount and Tammy Abraham, brought back two young players who were on loan. Andreas Christensen is an incredibly interesting player.
“For the first time in a long time, you have a team with which the fans can really identify. If a Timo Werner or possibly a Kai Havertz were to be added, it would be an incredibly exciting and interesting story.
“If I think about where I see the clubs now and in two or three years, the decision for Chelsea would be a relatively easy one for me as a player.”