The Uruguayan squandered a number of chances in the 2-0 Europa League defeat at Zenit St Petersburg, but produced a fine individual display against Swansea on Sunday.
"He just wants to help the team so much," he said.
"If you look at the guy, coming back after the game at Zenit, when he could have been disappointed having had some chances - he is a real warrior of spirit, and he is a winner.
"He didn't want to come off. He wanted to stay on and get more goals.
"I've obviously got to think of the bigger picture.
"I wanted him to get a round of applause from the crowd because he deserved it, he worked his socks off.
"He is going to be very important for us tomorrow night. Giving him that little breather - at the time it is difficult for him to accept because he wanted to play - but he is a critical player for us."