Manchester United held a moment of silence for the victims of the arena attack during practice Tuesday ... and have vowed to help emergency services "in any way that may be required."
At least 22 people have been killed and 59 others were injured by the nail bomb attack.
Rooney's current and former teammates voiced their reactions on Twitter this morning.
Rooney: "Devastating news this morning. Thoughts and prayers are with all those affected."
Pogba: "My condolences to all families of the victims in Manchester and to all families who are losing loved ones in so many countries due to acts of violence."
Danny Higginbotham: "I'm a proud Manchester lad. What happened last night was truly horrific but won't break the great city. Thoughts with all affected"
Ashley Young: "Absolutely shocked to hear the news this morning. Thoughts with all that lost loved ones & prayers for those being treated in hospitals"
Daley Blind: "Can't believe what happened last night, My thoughts go out to all those affected. #PrayForManchester"
Man. United plays in the Europa League final against AFC Ajax on Wednesday, but UEFA says "there is currently no specific intelligence which might suggest that any of the UEFA Europa League final activities in Stockholm may be the target of attacks."
"UEFA has been closely working with local authorities and the Swedish FA for many months and the terrorist risk had been taken into account since the very beginning of the project."