Per the Daily Mail's Dominic King, Toffees manager Ronald Koeman said:
It's really a big injury for the boy. He had a scan last Monday, and from that scan it became clear it's the cruciate ligaments. Maybe there is something more in his knee but they need to control it now and do a scan again to see about the state of the injury.
It is a big, big disappointment. It's a big loss for everybody. First for the player, then for his team-mates and the whole club. He's an important player and it's a long-term injury. He is important for the team with his pressing and his pace.
Bolasie has seen two specialists regarding the injury and is expected to undergo surgery before Christmas.
Despite the setback, the 27-year-old appears to be keeping in high spirits:
Bleacher Report UK's Matt Jones and Jack Lusby are less optimistic, however:
The injury is a blow both for him and Everton—he's likely to just be a few months shy of his 29th birthday upon his return, and there's no telling whether he'll possess the same explosive pace that's so vital to his game when he does.
Bolasie had been enjoying a fairly strong start to his career at Goodison Park after arriving from Crystal Palace in the summer, bagging a goal and four assists in the Premier League.
According to Squawka, the winger completed 38 take-ons in just 13 games for the Toffees and created 20 chances—only Ross Barkley and Romelu Lukaku have laid on more opportunities for their team-mates.
After losing such an important player, Koeman confirmed he would be looking to sign a replacement in January. According to King, he could look to Manchester United's Memphis Depay—a report echoed by Samuel Luckhurst of the Manchester Evening News:
In the meantime, Aaron Lennon and Gerard Deulofeu will have to fill the gap left by Bolasie's absence.
The former is the closest to Bolasie in terms of pace, and he netted five Premier League goals last year, while the latter racked up four goals and 13 assists in all competitions. The Spaniard has struggled to show such form under Koeman but clearly possesses talent and potential in abundance.
The pair will need to help Everton get through the festive period, and each will have to make a case to be the first-choice replacement for Bolasie, but even if they perform well, a dip into the transfer market seems likely.