Fulham manager Claudio Ranieri said manager like Jose Mourinho and Antonio Conte are the "heart of football" and have to come back soon.
Mourinho's two-and-a-half-year tenure as United manager came to an end on Tuesday, a 3-1 defeat to Premier League leaders Liverpool proving to be the final straw for the club in what has been a frustrating season.
Conte was let go back in July, having won a Premier League tile at the club the year before, a fate that has also fallen on both Mourinho and Ranieri in the past.
Mourinho has already been linked with a return to Real Madrid with Conte’s name having been floated for the post as well, and Ranieri believes both bosses should return to the game soon.
"I send a message with him. We are very friendly," Ranieri told reporters of Mourinho.
"I think a man like Mourinho has to come back soon, so too Antonio Conte. These managers are the heart of football.
"They win in every country. It's good for all the managers to see them on the pitch."
Mourinho was very supportive of Ranieri following his sacking by Leicester City, having led the Foxes to the Premier League title in shocking fashion.
“From my experience there are two things Claudio needs to know,” Mourinho said at the time.
“The first is that nobody can delete history. You cannot go to the laptop and press delete. And I can promise him also that nobody forgets.
Manchester United (@ManUtd) December 20, 2018
“Leicester fans will always be positive about his time at the club. They will react in a positive way.”
Ole Gunnar Solskjaer has been appointed caretaker boss until the end of the season as United search for a permanent replacement.
Tottenham manager Mauricio Pochettino has been strongly linked with taking over in the summer, but Solskjaer will be tasked with trying to climb back into the fight for the Champions League places in the Premier League.
The Old Trafford outfit are sixth and 13 points back of Chelsea, who currently sit fourth.
And Solskjaer will be thrown into the fire quickly. Cardiff City, Huddersfield and Bournemouth are all on tap for Manchester United before the turn of the year.