Rebic has been a major part of Croatia's World Cup campaign, which has seen them reach the final after a 2-1 win over England last night, and his impressive performances have reportedly caught the eye of a number of teams.
Speaking to Sport1, via the Manchester Evening News, Frankfurt sporting director Fredi Bobic admitted that he would be willing to let Rebic go if the right offer came in.
He said: "I am very happy about the success of Ante Rebic with the Croatian national team. The players and the coaching team are watching the World Cup games together in the training camp in the USA and, of course, are especially excited about Ante.
"So far, there are no offers available and we're planning a steady season with Ante for the new season.
"Of course we know that a lot can happen when a big club makes an offer. We cannot compete economically with the big ones, but that's nothing new."
Rebic has played in five of Croatia's six games so far in the World Cup, scoring one goal in his team's 3-0 win over Argentina.
The 24-year-old had an impressive season in the Bundesliga, scoring six goals in 25 appearances. He also scored two goals in the DFB-Pokal Cup final in a 3-1 against Bayern Munich, helping Frankfurt win their first DFB-Pokal trophy in 30 years.
Manchester United have already made two signings in the transfer market so far this summer, managing to secure deals for midfielder Fred from Shakhtar Donetsk and full-back Diogo Dalot from Porto.