Barcelona's €30m option to make Rashford's loan permanent lapsed without being triggered, ending a season-long spell in Catalonia during which the 28-year-old scored 14 goals and provided 11 assists in 49 appearances as Barcelona won the La Liga title.
The club instead used its attacking budget this summer to sign Anthony Gordon from Newcastle United for around £70m, effectively closing the door on a permanent Rashford deal despite reports that Flick himself had remained keen to keep him.
Speaking after Barcelona's pre-season friendly against Birmingham City, Flick addressed the decision not to sign Rashford directly.
"What I can say about Marcus is I appreciate a lot to work with him. Sometimes it's not always that you do know what happens with players who are on loan.
"I think our situation is not easy for that," he said.
"But I appreciate working with him a lot; he's a fantastic player and a fantastic person. I think also the team miss him, I will miss him, but it's life, this is what we have to accept."
Rashford's return to Old Trafford comes after a turbulent period at United, having previously spent the second half of the 2024-25 season on loan at Aston Villa following a breakdown in his relationship with Ruben Amorim.
He is currently on holiday following England's involvement in this summer's World Cup and is expected back at Carrington in early August, with his long-term future at United still far from settled given the circumstances that led to his exit in the first place.
Michael Carrick, now in charge at Manchester United, is understood to be assessing his options for the forward line this summer, with reports linking the club to alternatives such as Ollie Watkins and Dusan Vlahovic — speculation that has only added to uncertainty over whether Rashford will be given a route back into the first-team picture or moved on again before the transfer window closes.
Barcelona, for their part, have made clear their door has not been permanently shut on a return in future, with sources suggesting any renewed interest would need considerably more favourable terms than the option they let lapse this summer.