The England forward is enduring his worst season as a United player, managing just five goals for the campaign.
Rashford was an unused substitute for the 3-0 win over Brentford on Monday, and Keane thinks there are deeper problems for the United academy graduate.
Speaking after Monday's game, Keane said: "Rashford has stopped running in behind. He's certainly short on confidence. We've seen his quality before but we've not seen it for a year or two now.
"He's taken his eye off the ball with all the off-the-field stuff, and that's why I keep an eye on my young players all the time. Rashford's been around for a few years now and he's got that experience, he's played international football.
"He doesn't mix his game up enough, he doesn't look like he's matured. We've seen a few games recently where, even when he's playing at Liverpool through the middle, he played like a child
"He's not learned about the game. He's just drifting. He's lost the hunger – these young players get a lot of money and good luck to them but he's lost the eye of the tiger and he's struggling to get it back.
"Wherever Man United want to go, Marcus has been left behind. He's left out again tonight – he must be sitting on the bench thinking, "where has it all gone wrong?" He's got it back - he's shown it before - but sometimes, when you do lose it, it is hard to get it back.
"He's got to get the hunger back into his game and make runs like we've seen from young Elanga tonight. Players like him are a bit more innocent and hungry to succeed. Rashford has lost that bit of hunger, which is huge for a player. He's a talent but we're not seeing it."
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