Rasmus Hojlund scored a brilliant opener on his 21st birthday before Alejandro Garnacho added a second-half brace to equal the club’s best Premier League win of the season.
Ten Hag was all smiles after full-time, knowing his side had leapfrogged West Ham into sixth, six points behind the Champions League spots.
However, Martinez pulled up with what appeared to be a knee issue following a challenge against Vladimir Coufal just before the 70-minute mark.
The 26-year-old attempted to continue but went to ground moments later before United replaced him with Raphael Varane.
Martinez walked down the tunnel with the assistance of United’s physio in a promising indicator that the injury wasn’t too serious.
But Ten Hag has sparked fear with his admission after the match, claiming he is joining fans in fearing the worst based on the early signs.
“Lisandro Martinez’s injury does not look good. It is very bad for him. We wish Licha all the best. It’s very bad also for the team,” Ten Hag revealed.
“It seems bad. We have to wait and make the right diagnosis. At this moment, we are very sad.
“Hopefully, it is not too bad, but we can only pray. It looks like he has to sit some games out injured again.”
He added: “I can’t tell you the details. We have to find out in the next few days what the diagnosis is. He is just returning. He is very important for us. He brings spirit to the team, aside from his football ability.”
Martinez had only just returned from four months away after undergoing a second surgery on a foot injury that ended his 2022/23 season.
He featured for the first time since the 4-3 defeat at Bayern Munich by coming on as a substitute in United’s 2-2 draw with Tottenham last month.
Martinez then started against Newport County, Wolves and West Ham, emphasising his importance to Ten Hag’s plans.
But the Argentine could face another stint on the injury table if the Dutchman’s concerns are with good reason.
With away trips to Aston Villa, Luton Town and Manchester City to come in United’s next four Premier League games, it comes at a worrying time.