Guus Hiddink believes the loss of Diego Costa to injury in the second half proved costly as Chelsea were knocked out of the Champions League at the last-16 stage by Paris Saint-Germain on Wednesday.
Spain striker Costa put in a hugely impressive performance in the first half at Stamford Bridge and scored the equaliser to cancel out Adrien Rabiot's 16th-minute opener for the visitors.
However, Costa - who was missing in Saturday's 1-1 Premier League draw against Stoke City - suffered a recurrence of a hip injury and was replaced by youngster Bertrand Traore on the hour mark.
PSG sensed an opportunity and the evergreen Zlatan Ibrahimovic - sent off against Chelsea at the same stage last season - gained redemption by turning in Angel Di Maria's cross in the 67th minute to secure a 2-1 win and 4-2 aggregate triumph for the Ligue 1 champions.
"Yeah, he [Costa] made a beautiful goal of course, but we knew he was not totally fit," Chelsea boss Hiddink told BT Sport. "He was desperate to play, he did what he could but was not fully fit.
"You saw the difference between the squads when we had to bring young guys on and they have Edinson Cavani to come in. Although, I must say that we had young Kenedy, making a [full] Champions League debut at left-back and he was excellent.
"We'll see [how serious Costa's injury is]. I hope it is not long term for Diego. I must pay respect for him, he desperately wanted to play. He had a very good game."
Chelsea's defeat means their last hope of silverware this season is in the FA Cup, while their chances of qualifying for next season's Champions League are increasingly slim as they are 10 points adrift of fourth-placed Manchester City in the Premier League.
"We knew it was a difficult season for Chelsea when I came in last December," Hiddink added. "We were panicking because we were in relegation danger.
"Chelsea's target was to get out of relegation which we did in short space of time, but it is difficult to get fourth place and the Champions League."