AFTER UPDATE trigger using an aggregate from the updated table

I'm having trouble with an update trigger. I want the trigger to set Quarterbacks.Yards equal to the sum of Wide receiving yards if they're on the same team.

create trigger T_Receiving_Passing
on NFL.Widereceivers
after update
as
begin
declare
@receivingyards int,

select @receivingyards = sum (Widereceivers.Yards) from NFL.Widereceivers
update NFL.Quarterbacks
set Quarterbacks.Yards = @receivingyards
where Quarterbacks.Team = Widereceivers.Team
end

At the end of the statement, Widereceivers.Team is underlined in red, and it is causing errors. I get this same error whenever I try to reference a column in another table without naming the table in a from clause. How can I fix this problem?