Rebic, 24, was one of the breakout stars at the World Cup and scored in Croatia's Group D victory over Argentina on their route to the final against France.
"Some are first-team, some under-23s, some coming from loans and going on loan, some are younger, under-18s".
"I don't think it's about us getting the best out of him, it's about him giving the best he has to give", declared the Red Devils boss.
Jose Mourinho has been relatively quiet in the transfer market with third-choice goalkeeper Lee Grant and young full-back Diogo Dalot the only arrivals other than Fred with the Brazilian worldwide the only one of the trio expected to be a first team regular.
United open their campaign at Old Trafford against Leicester City on Aug 10 before travelling to Brighton on Aug 19 for their second fixture.
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Champions League holders Real Madrid have already made enquiries for the highly rated star. I think it was the third goal, he couldn't do it anymore.
Manchester United continues their pre-season on Wednesday when they host AC Milan in Pasadena, following draws with Club America and San Jose Earthquakes so far. So against Leicester and Brighton the situation is not wonderful for us.
When you're still starting Chris Smalling and Phil Jones as a central defense pairing in a cup game, it's time for an upgrade.
Marcos Rojo is one player who may not join up with Mourinho's squad in the US. "This is what you work towards, the start of the season, during pre-season".
"I think it was on my personal level needed to play another season regularly and it was good for me as well and I think it's good for the team and the club that I'm more prepared now than a year ago", he added.
"Man City and Tottenham in a similar situation as we are, but other clubs are in a different situation".