"I want to wait", Mourinho said when asked about the injury status of the duo. "But the news is fairly negative I think", United manager Jose Mourinho told BT Sport.
But with United into the Europa League semifinals and fighting for a place in the Premier League top four, Mourinho needs Rashford to become "the man" because top United scorer Zlatan Ibrahimovic (28 goals) suffered a nasty-looking knee injury in the final minute of normal time against Anderlecht, the Belgian league leaders.
UNITED beat Anderlecht after extra-time tonight - here's some of the best reaction from their fans.
Mourinho last week said the England centre-backs were out until mid-May, but suggested after the Anderlecht win that they need to be "brave".
Ibrahimovic, 35, has been a sensation since joining on a free transfer from Paris Saint-Germain a year ago, scoring 28 goals and inspiring United to victory in the League Cup. But with a special spirit, you can (come back) sooner than expected.
The counter-attacking move was started by Paul Pogba, whose towering header was moments later repelled by Ruben before the on-loan Deportivo La Coruna stopper thwarted Jesse Lingard's curling effort.
Anderlecht levelled shortly after the half hour mark as Hanni lashed home following a deflected left-footed strike from Youri Tielemans, which rattled the crossbar.
Rashford can expect some more chances to shine before the end of the season after Zlatan Ibrahimovic went down with a serious injury during Thursday evening's match.
Freed by Michael Carrick, Ibrahimovic shanked wastefully wide.
The Europa League final takes place on 24 May (19:45 BST) at the Friends Arena in Stockholm, Sweden.
Leon Goretzka and Guido Burgstaller scored in the space of three second-half minutes to cancel out Ajax's advantage before Daniel Caligiuri hit Schalke's third after 101 minutes thanks to Sead Kolasinac's fine cross.
United are joined in the last four by Celta Vigo, who beat Mat Ryan's Genk, Ajax and Olympique Lyon.
But Nick Viergever quickly levelled on aggregate before Amin Younes hit a last-minute victor for the Dutch who finished the game with 10 men after Joel Veltman had been red-carded in the 80th minute.