Costa, 25, is set for a £32m transfer from Atletico Madrid, joining fellow Spaniard Cesc Fabregas in west London following his £30m move from Barcelona last week.
The arrival of such a prolific striker, who scored 35 goals last season, would greatly increase the pressure on Torres to win a place in Jose Mourinho’s side.
But Torres said: “The level of competition at big clubs should always be very high. People have been saying for some time that Chelsea have signed a striker, and if that’s Diego then it's very welcome.”
Former Arsenal star Fabregas, part of Spain’s World Cup squad along with Torres and Costa, moved to Chelsea after requesting to leave Barcelona and being told by Arsene Wenger he would not be taking the option to resign the midfielder.
And Torres said: “I wish he could have joined us at Chelsea a couple of years earlier. He is a blessing for strikers; always looking to find that final, key pass. He's the type of player Chelsea don't have.”
Torres, 30, featured infrequently under Mourinho last season, scoring 11 goals, but is expected to remain at Chelsea, who did not retain Samuel Eto’o following the expiry of his one-year deal.
Chelsea have also seen Belgium striker Romelu Lukuku return to the club following a successful season-long loan at Everton, where he scored 18 times.