Jose Mourinho is unsure about the severity of the hamstring injury sustained by Paul Pogba during Manchester United's straightforward, if not completely satisfactory, Champions League return. Tuesday's Old Trafford encounter with Basle ended their 644-day absence from European football's top table and it was as comfortable a reintroduction as they could have hoped for.Substitute Marouane Fellaini headed United into a deserved first-half lead and compatriot Romelu Lukaku followed suit early in the second period with his first goal in the Champions League group stage.Marcus Rashford came off the bench to net his first goal in the competition and wrap up a 3-0 win against Basle on a night when Pogba's hamstring complaint took the gloss off the Group A victory.An evening that started with him leading United out as captain for the first time was brought to an halt inside 19 minutes, with the 24-year-old reportedly seen leaving Old Trafford on crutches."It's three important points in the first match at home," Mourinho said."Now we finish at home (to CSKA Moscow), too, which gives us a chance to play here in case if we need these points to qualify, but in between we have difficult matches away from home."But to start with three points at home is always important."I don't know honestly (what is wrong with Pogba) but by experience, just by looking and feeling it, I think it is a hamstring."Big? Small? I don't know, but a hamstring for sure."Mourinho will be sweating on the fitness of his star midfielder in a tough September that includes a trip to CSKA Moscow on top of domestic exertions.The Russian side secured a shock 2-1 win at Benfica on Tuesday evening, while over at Old Trafford some of United's second-half play frustrated their manager."We played quite well but after 2-0 everything changed," Mourinho told BT Sport."We stopped to play, we stopped to think, we stopped to play seriously, we stopped to make the right decisions on the pitch."Bad decisions, fantasy football, PlayStation football, tricks and when you stop to play as a team I don't like, and you gamble a bit."Probably the players felt the game was under control at 2-0 but football is football, you have to respect opponents."United's win was one of a number of large scorelines around Europe on first night of the Champions League group stage, with Paris St Germain, Bayern Munich, Chelsea and Barcelona securing comprehensive wins.Asked if smaller sides were getting weaker of the big teams more powerful, Mourinho retorted: "I think the Champions League for these teams that are you speaking (about) and for the Real Madrids, and the Barcelonas, and Bayerns, I think the Champions League starts in February."Now is just the warming up and in February when us English teams are trying to survive after the winter period, they are fresh and ready after this warm-up."For them, this is a warm-up. I think we are in the second level and the second level is let's qualify, let's make the points to qualify for the knockout phase."When you do that, if you do that, let's enjoy to play against the big guys."Basle have enjoyed playing against the big boys in years gone by, drawing their previous two visits to Old Trafford.Tuesday was an entirely different matter for Raphael Wicky's side, whose side lost a fourth straight game in all competitions."Losing 3-0 to Manchester United is not something you can be happy with, but I'm not disappointed with my team, I'm disappointed by the result," the Basle head coach said."We can learn from this, and this is what I told me team at the final whistle. This team was suffering together, fighting together. What I didn't like at the beginning was I saw a team that was quite scared."