Carlo Ancelotti admitted Chelsea cannot afford another slip-up if they want to stay in the title race.
Chelsea dropped to the bookies' third-favourites behind both leaders Manchester United and Arsenal after a 1-1 draw at Blackburn.
Victory over Portsmouth on Wednesday will see Chelsea pull back to within a point of United but Ancelotti admitted the result at Blackburn was disappointing.
"It's important to win on Wednesday," said the Chelsea manager.
"We have less probability to win this title but we want to give it our best.
"We need to work together, stay compact and the next game against Portsmouth is very important, we can stay close to Manchester United.
"I don't know how many points we need to win the title but I know we have to win on Wednesday and then prepare game by game."
Chelsea had gone in front against Blackburn through Didier Drogba's 22nd league goal of the season but let the lead slip and were punished by El-Hadji Diouf's header late in the second half.
Chelsea have dropped out of the top two for the first time since the second weekend of the season and are now four points behind Manchester United and two behind Arsenal, though with a game in hand.
Ancelotti added: "It will be more difficult now to win the title but it's not impossible.
"We are disappointed because I think we lost two points here, and important points.
"We had prepared well for the game, we had tried to win to cancel out the game against Inter and for one hour we did well.
"It's not an easy moment for us and we have to maintain our confidence and our composure.
"Manchester United are now top of the league and we have to run more and do better and leave this behind us as soon as possible."
Chelsea are likely to be without Branislav Ivanovic against Portsmouth after he suffered a knee injury but keeper Petr Cech and Michael Ballack should be available.
Ancelotti admitted his players had "lost composure" against a rugged Blackburn side whose determination was summed up by an awesome league debut by central defender Phil Jones, who only turned 18 last month.
Blackburn boss Sam Allardyce said: "It was a great performance by a young man who is looking like he has a bright future.
"As big a team as Chelsea are we felt it was time to give him the chance.
"It's nice to see a player that excites you like that - especially when he's a centre-half like I was.
"We have recommended him to the England under-19s and said he should be playing at that level but there is still a long way to go for him to be a regular Premier League centre-half."