The Spain striker was back to his best on Wednesday night as the 28-year-old bullied the Paris Saint-Germain defence at Stamford Bridge.
Costa equalised for Chelsea after Adrien Rabiot’s opener in the second leg of their Champions League last-16 clash before he was forced off with a hamstring injury in the second half.
Guus Hiddink’s side went on to lose 2-1 without Costa after Zlatan Ibrahimovic secured a 4-2 aggregate lead for the Ligue 1 side.
Speaking to ITV Sport on Wednesday night following Chelsea’s Champions League exit, former Manchester United captain Keane explained why Costa is a frustrating player.
”It must be frustrating for the club,” Keane told ITV Sport.
“We’ve seen the best and worst of him tonight. He always seems to be injured. He never seems to be 100 per cent fit. That is a major problem for an important player at your club.”
Costa has scored 11 goals in 15 games since Hiddink replaced Jose Mourinho as Chelsea manager in December.
The Spain international moved to Chelsea in a £32m deal from La Liga giants Atletico Madrid in 2014.
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